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Pearson's product moment correlation coefficient, or Pearson's r was developed by Karl Pearson (1948) from a related idea introduced by Sir Francis Galton in the late 1800's. In addition to being the first of the correlational measures to be developed, it is also the most commonly used measure of association. All subsequent correlation measures have been developed from Pearson's equation and are adaptations engineered to control for violations of the assumptions that must be met in order to use Pearson's equation (Burns & Grove, 2005; Polit & Beck, 2006). Pearson's r measures the strength, direction and probability of the linear association between two interval or ratio variables.
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Running head: PEARSON'S PRODUCT MOMENT CORRELATION
Pearson's Product Moment Correlation: Sample Analysis
Jennifer Chee
University of Hawaii at Mānoa School of Nursing
Introduction
Pearson's product moment correlation coefficient, or Pearson's r was developed by Karl
Pearson (1948) from a related idea introduced by Sir Francis Galton in the late 1800's. In
addition to being the first of the correlational measures to be developed, it is also the most
commonly used measure of association. All subsequent correlation measures have been
developed from Pearson's equation and are adaptations engineered to control for violations of
the assumptions that must be met in order to use Pearson's equation (Burns & Grove, 2005; Polit
& Beck, 2006). Pearson's r measures the strength, direction and probability of the linear
association between two interval or ratio variables.
Pearson's Product Moment Correlation Coefficient - Pearson's r
Pearson's r is a measure of the linear relationship between two interval or ratio
variables, and can have a value between -1 and 1. It is the same measure as the point-biserial
correlation; a measure of the relationship between a dichotomous (yes or no, male or female) and
an interval/ratio variable (Cramer, 1998).
The advantage of using Pearson's r is that it is a simple way to assess the association
between two variables; whether they share variance (covary), if the relationship is positive or
negative, and the degree to which they correlate. The disadvantages of using Pearson's r is that it
can not identify relationships that are not linear, and may show a correlation of zero when the
correlation has a relationship other than a linear one. Additionally the types of variable that can
be evaluated are limited.
In addition to Pearson's r, semipartial and partial correlation can be employed in order
to estimate the relationship between an outcome and predictor variable after controlling for the
effects of additional predictors in the equation.
. "Pearson's correlation is the ratio of the variance shared by two variables" (Cramer,
1998, p. 137). A means of illustrating correlation is with a Venn diagram (Figure 1). Each circle
represents the amount of variance of each variable. In Figure 1a the circles do not overlap,
indicating that there is no correlation between the two variables. In Figure 1b the two circles
overlap, the size of the correlation is reflected in the degree of overlapping area. In a perfect
correlation (Figure 1c) the circles would overlap each other completely.
There are multiple formulae that can be used to compute Pearson's r :
For small samples, Pearson's r may be calculated manually using:
!=
!!Σ!" − Σ! Σ!
[!Σ!!! – Σ!! ] [!Σ!!! −! Σ!! ]
Where:
r = Pearson's correlation coefficient
n = number of paired scores
X = score of the first variable
Y = score of the second variable
XY = the product of the two paired scores
Or similarly:
!=
covariance!of ! variable!! !and !!
variance!of! variable!!!×! variance!of ! variable!!
!
Analogously:
!=
covariance!of! variable!!!and !!
!"#$%#&%!!"#$%&$'( !!" ! !"#$"%&'!! !× !( !"#$%#&%!!"#$%&$'( !!" ! !"#$"%&'!!) !
"The larger the covariance is, the stronger the relationship. The covariance can never be bigger
that the product of the standard deviation of the two variables" (Cramer, 1998, p. 139).
The denominator in the equations is always positive, however the numerator may be
positive, zero, or negative, "…enabling r to be positive, negative, or zero respectively" (Zar,
1999, p. 378).
Assumptions
Of Primary importance are linearity and normality. Pearson's r requires that interval data
be used to determine a linear relationship. Further assumptions must be met in order to establish
statistical significance, "…for the test statistic to be valid the sample distribution has to be
normally distributed" (Filed, 2009, p. 177). Homoscedasticity assumes that the error at each level
of the independent variable is constant. A violation of the assumption of homoscedasticity
increases the chances of obtaining a statistically significant result even though H0 is true.
Significant Testing of Pearson's r
In order to infer that the calculated r is applicable to the population from which the
sample was drawn, statistical analysis must be performed to determine whether the coefficient is
significantly different from zero.
Th e hypothesis that the population correlation coefficient is 0, and may be computed by
calculating t :
!=
!!−2
1−!!
Where:
r = Pearson's product-moment correlation coefficient
n = sample size of paired scores
df = n - 2
If the sample size is small, a high correlation coefficient (close to -1 or 1), may be non-
significant. Contrariwise, a large sample may have a statistically significant r but have no clinical
significance. Subsequently it is important to consider both the magnitude of the correlation
coefficient, the significance of the t-test, and the context of the research question.
Coefficient of Determination
By squaring the correlation coefficient r, the total variability in Y can be accounted for
after regressing Y on X; r 2 can be considered to be a measure of the strength of the linear
relationship. The resulting value when multiplied by 100 results in a percent variance, e.g., if the
correlation coefficient for X and Y is r = .50, then r2 = (.50)(.50) = .25 = .25(100) = 25%. X
explains 25% of the variability in Y (Zar, 1999).
The Matrix
The output from a statistical program such as SPSS contains a single table, the
correlation matrix. The correlation analysis computes the correlation coefficient of a pair of
variables - or if the analysis contains multiple variables - the matrix contains the results for all
variables under consideration, and is obtained by performing bivariate correlational analysis on
every possible pairwise combination of variables in the dataset. "On the matrix, the r value and
the p value are given for each pair of variables. At a diagonal through the matrix are variables
correlated with themselves" (Burns & Grove, 2005, p. 488), the value of r for these correlations
is equal to 1 and the p value is .000 because a variable is perfectly related to itself.
Pearson's Partial Correlation
A statistically significant association between two measures does not mean that the two
variables have a causal relationship, e.g. if students who spend more time in the simulation
laboratory were found to have higher scores during skills evaluations, this association does not
necessarily imply a causal link between hours in the 'sim lab' and higher marks on evaluations,
it is conceivable that the relationship is specious (Zar, 1999). Such relationships are referred to as
spurious. Such spurious associations are the result of one or more other factors that are
"genuinely related to these two variables" (Cramer, 1998, p. 156). To continue the example, a
student's curricular progression may be related to both hours in the 'sim lab' and higher marks
on evaluations. When curricular progression is taken into consideration there may be no
association between the other two variables. A more likely example of a spurious relationship is
one in which part but not all of the association between two measures may be due to one or more
other factors, e.g., students who spend more time in the simulation laboratory may have had
more exposure to nursing skills secondary to their progression through the curriculum. If the
influence of the student's position within the curriculum on the association between hours in
'sim lab' and higher marks on evaluations is partialled out, it can be presumed that the remaining
association between these two variables is not a direct result of position within the curriculum.
Similarly, removing the influence of hours in 'sim lab' from the relationship between position
within the curriculum and higher marks on evaluations to determine how much of the
relationship between the two variables was not due to hours in 'sim lab'.
It is important to note that a significant reduction in the size of an association between
two variables by partialling out the influence of a third variable does not necessarily
mean that that reduction indicates the degree of spuriousness in the original relationship
between the two variables. (Cramer, 1998, p. 157)
To perform a partial correlation to control for one variable – first -order partial
correlation ( r12.3 ) the following formula may be used:
!
!".!=
!
!" −(!
!"!×!!!
!")
(1−2!"
!)!× !(1−!2 !"!
!)
To perform a partial correlation to control for two variables – second-order partial
correlation ( r12.34 ) the following formula, which is based on the formula for first order
correlations, may be used:
!
!".!=
!
!".!−(!
!". !!×!!!
!". !)!
1−2 !".!
!)!×!(1−!2 !".!!
!)
And third-order partial correlations are similarly "based on second-order partial
correlations and so on" (Cramer, 1998, p. 160; Pedhazur, 1982).
Pearson's Semipartial Correlation
Whereas partial correlation allows for partialling out a variable from two other variables,
semipartial correlation (or part correlation) allows for the partialling out of a variable from only
one the variables that are being correlated. Put another way, semipartial correlation is the
correlation between the outcome variable and another predictor variable that has been controlled
for by another predictor variable. For example assume an admissions committee at a school of
nursing is considering three variables, high school grade point average (HS_GPA), score on the
Test of Essential Academic Skills (TEAS), and intelligence quotient (IQ). It would not be
unreasonable to assume IQ and TEAS are positively correlated. In this situation the admissions
committee is interested in the relationship between TEAS and HS_GPA, while controlling for
IQ. The formula for r12.3 (correlation of HS_GPA and TEAS while controlling for IQ) could
provide some information. Also r13.2 will indicate the correlation for IQ and HS_GPA while
controlling for TEAS.
It is possible however, that of greater interest to the [admissions committee] is the
predictive power of the [TEAS] after that of [IQ] has already been taken into
account…the interest is in the increment in the proportion of the variance in [HS_GPA]
accounted for by the [TEAS] that is over and above the proportion of variance accounted
for by [IQ]. In such a situation [IQ] should be partialled out from [TEAS], but not from
[HS_GPA] where it belongs [by performing a semipartial correlation.] (Pedhazur, 1982,
p. 115)
Sample Problem
The flexibility permitted by simulation has made it attractive to institutions as a
supplement to real-world clinical experience (Murray, Grant, Howarth, & Leigh, 2008; Rosseter,
2011). Resource constraints by academic institutions, fiscal reforms and societal pressures have
promoted a safety-conscious-litigation-avoiding culture where simulation provides a means of
risk free applied practice (Health care at the crossroads: Strategies for improving the medical
liability system and preventing patient injury, 2005).
Effective use of simulation is dependent on a complete understanding of the role of
simulation's central conceptual elements. Deliberate practice, a constituent of Ericsson's theory
of expertise, has been identified as a central concept in effective simulation learning (Issenberg,
McGaghie, Petrusa, Gordon, & Scalese, 2005).
Deliberate practice is compatible with simulation frameworks already being suggested for
use in nursing education (Jeffries, 2006). In a review of the websites of leading nursing schools
in the United States ("Top Nursing Schools and Best Ranked Nursing Colleges," 2011), each
mentioned access to simulation centers or labs. In an informal non-scientific survey of these
same schools, respondents reported use of low, moderate, and high fidelity simulation in addition
to task trainers and moulage/tabletop models. The respondents reported that, depending on the
students' class standing, they are required to complete anywhere from 100 – 280 real-world
clinical hours, and a minimum of 4 simulated clinical hours (.04 and .014% respectively) in the
simulation lab during adult health medical/surgical semesters. The same institutions describe
simulation as a compliment to rather than a replacement for clinical experience (Chee, 2011).
A small undergraduate-nursing program was considering a substantial increase in the
amount of curricular time the nursing students spent in the simulation laboratory ('sim lab').
Before making the changes the school wished to evaluate the impact of simulation on the
biannual skills evaluations. The 'sim lab' was made available to the students on a voluntary
basis. The 'sim lab' was equipped with evidence-based multimedia learning modules for the
purpose of independent deliberate practice of clinical nursing skills.
Data was collected regarding the number of hours each of the undergraduate-nursing
students spent in the simulation laboratory participating in deliberate practice, pre- and posttest
evaluation scores, and some demographic information were collected. The course instructors
hypothesized that there was no difference in the evaluation scores depending on the amount of
time students spent participating in deliberate practice for the purpose of learning nursing skills.
SPSS 21 was used to perform all assumption testing and analysis.
Null and Alternative Hypothesis
The null and alternative hypothesis for the correlation is:
H0 : ρ = 0, the population correlation coefficient is equal to zero
And the alternative hypothesis is:
HA : ρ ≠ 0, the population correlation coefficient is not equal to zero.
Where ρ is the population correlation coefficient
Results
Testing of Assumptions
A Pearson correlation is appropriate only when a linear relationship exists between the
two variables. To determine if a linear relationship existed between the primary variables of
interest a scatter plot (Figure 2) was generated between the predictor (sim_hours) and the
outcome variable (post_test). The relationship approximated a straight line; the scatterplot
suggests that the relationship of the two variables is positive and linear; the assumption of
linearity has not been violated.
Next the assumption of bivariate normality was evaluated. Both variables were tested for
normality using the Shapiro-Wilk test (Table 1), which was appropriate for the sample size. Both
variables are normally distributed, as assessed by Shapiro-Wilk's test (p > .05).
Pearson's Product Moment Correlation Coefficient
The main analysis was performed with sim_hours as the predictor variable and post_test
as the outcome variable (Table 2). There was a positive correlation between hours spent
practicing in the 'sim lab' and post-test scores, r(98) = .751, p < .0005. The relationship was
statistically significant; therefore we reject the null hypothesis and accept the alternative
hypothesis; there is a correlation between time spent in the 'sim lab' and the successful execution
of psychomotor skills in a posttest evaluation.
Strength of Association
The absolute value!of the Pearson coefficient determines the strength of the correlation.
Using Cohen's (1988) guidelines for interpreting strength of association (.1 < !! < .3 a small
correlation, .3 < !! < .5, a moderate correlation, !>. 5 a strong correlation), the results of this
analysis show a strong positive correlation between the predictor and outcome variables.
Coefficient of Determination
Hours spent in the 'sim lab' explained 60% of the variability in post-test scores: r 2 =
(.751)(.751) = .564 = .564(100) = 56%.
Partial Correlation
The instructors suspected that the correlation between time spent in the 'sim lab' and
posttest evaluation scores was only a spurious correlation caused by a difference in the baseline
characteristics of the students. Subsequently they investigated the relationship of the variables
sim_hours and post_test further by evaluating the influence of other data that had been collected.
A second correlational analysis was performed (Table 3) which included a second
predictor variable, pre_test, suspected of having a possible influence on post_test.
From this matrix (Table 3) it was known that post-test scores are strongly positively
correlated with hours spent in the sim_lab, and it was also known that post_test scores were
highly positively correlated with pre_test . But what was unknown was what the correlation
would be when pre_test was controlled for.
The evaluation was accomplished with a partial correlation. The output (Table 4)
displayed the zero order correlation coefficients, which were the three Pearson's correlation
coefficients without any control variable taken into account. The zero-order correlations
supported the hypothesis that more hours spent in the 'sim lab' increased the post-test score. A
strong association of r = .751, which was highly significant. The variable pre_test was also
significantly correlated with both sim_hours and post_test score (r = .665 and r = .927). The
second part of the output showed the correlation coefficient between sim_hours and post_test
when pre_test was controlled for. The correlation coefficient was now, r12.3 = .479, and was
statistically significant, p < .005.
Thus when the influence of the variable pre_test was controlled for the sim_lab
accounted for only 23% of the variability in post_test.
Discussion
The second (partial) correlation was useful in understanding the impact of factors other
than sim_hours on students' nursing skill evaluations. The instructors should continue to look at
elements of student experience and curriculum that contribute to pre- and posttest scores in an
effort to make intelligent decisions about how best to focus resources.
Conclusion
Pearson's product moment correlation coefficient (Pearson's r) is not conceptually
demanding; which underlies it's utility. It is one of the foundational elements of modern
statistical analysis, and is a simple means of evaluating the linear relationships between
variables. The direction (positive or negative) and strength of the relationship (coefficient of
determination) may also be evaluated. While it does not show causal relationships it is an
pragmatic way to understand the relationship of variables of interest.
References
Burns, N., & Grove, S. (2005). The Practice of Nursing Research: Conduct, Critique, and
Utilization (5 ed.). St. Louis: Elsevier Saunders.
Chee, J. (2011). [Simulation Use in Undergraduate Nursing Education].
Cohen, J. (1988). Statistical power analysis for the behavioral sciences. New York: Academic.
Cramer, D. (1998). Fundamental Statistics for Social Research. London: Routledge.
Crown, W. (1998). Statistical Models for the Social and Behavioral Sciences: Multiple
Regression and Limited-Dependent Variable Models. London: Praeger.
Issenberg, S., McGaghie, W., Petrusa, E., Gordon, D., & Scalese, R. (2005). Features and uses of
high-fidelty medical simulations that lead to effective learning: A BEME systematic
review. . Medical Teacher, 27(1), 10-28. doi: 10.1080/01421590500046924
Jeffries, P. (2006). Developing evidenced-based teaching strategies and practices when using
simulations. Clinical Simulation in Nursing, 2(1), e1-e2. doi: 10.1016/j.ecns.2009.05.014
Murray, C., Grant, M., Howarth, M., & Leigh, J. (2008). The use of simulatoin as a teaching and
learning approach to support practice learning. Nurse Education in Practice, 8, 5-8.
Pearson, K. (1948). Early Statistical Papers. Cambridge, England: University Press.
Pedhazur, E. (1982). Multiple Regression in Behavioral Research: Explanation and Prediction.
New York: Holt, Rinehart and Winston.
Polit, D., & Beck, C. (2006). Essentials of Nursing Research: Methods, Appraisal, and
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Rosseter, R. (2011). Nursing shortage fact Sheet: American Associaltion of Colleges of Nursing.
Zar, J. H. (1999). Biostatistical Analysis. Upper Saddle River, NJ: Prentice Hall.
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June Chang
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- Yasmin Ahmad
This study was conducted with the aim to identify the relationship availability facilities at home with compulsive Internet use among primary school student in Taiping Perak. This study use quantitative method. The questionnaire were distributed to 2 primary school and involve 100 primary school student in Taiping Perak as a sample in this study. The findings of this study indicates that there is a positive relationship between the availability of facilities at home and compulsive Internet use among primary school students.
1969 to 2003, 34 years. Simulations are now in widespread use in medical education and medical personnel evaluation. Outcomes research on the use and effectiveness of simulation technology in medical education is scattered, inconsistent and varies widely in methodological rigor and substantive focus. Review and synthesize existing evidence in educational science that addresses the question, 'What are the features and uses of high-fidelity medical simulations that lead to most effective learning?'. The search covered five literature databases (ERIC, MEDLINE, PsycINFO, Web of Science and Timelit) and employed 91 single search terms and concepts and their Boolean combinations. Hand searching, Internet searches and attention to the 'grey literature' were also used. The aim was to perform the most thorough literature search possible of peer-reviewed publications and reports in the unpublished literature that have been judged for academic quality. Four screening criteria were used to reduce the initial pool of 670 journal articles to a focused set of 109 studies: (a) elimination of review articles in favor of empirical studies; (b) use of a simulator as an educational assessment or intervention with learner outcomes measured quantitatively; (c) comparative research, either experimental or quasi-experimental; and (d) research that involves simulation as an educational intervention. Data were extracted systematically from the 109 eligible journal articles by independent coders. Each coder used a standardized data extraction protocol. Qualitative data synthesis and tabular presentation of research methods and outcomes were used. Heterogeneity of research designs, educational interventions, outcome measures and timeframe precluded data synthesis using meta-analysis. HEADLINE RESULTS: Coding accuracy for features of the journal articles is high. The extant quality of the published research is generally weak. The weight of the best available evidence suggests that high-fidelity medical simulations facilitate learning under the right conditions. These include the following: providing feedback--51 (47%) journal articles reported that educational feedback is the most important feature of simulation-based medical education; repetitive practice--43 (39%) journal articles identified repetitive practice as a key feature involving the use of high-fidelity simulations in medical education; curriculum integration--27 (25%) journal articles cited integration of simulation-based exercises into the standard medical school or postgraduate educational curriculum as an essential feature of their effective use; range of difficulty level--15 (14%) journal articles address the importance of the range of task difficulty level as an important variable in simulation-based medical education; multiple learning strategies--11 (10%) journal articles identified the adaptability of high-fidelity simulations to multiple learning strategies as an important factor in their educational effectiveness; capture clinical variation--11 (10%) journal articles cited simulators that capture a wide variety of clinical conditions as more useful than those with a narrow range; controlled environment--10 (9%) journal articles emphasized the importance of using high-fidelity simulations in a controlled environment where learners can make, detect and correct errors without adverse consequences; individualized learning--10 (9%) journal articles highlighted the importance of having reproducible, standardized educational experiences where learners are active participants, not passive bystanders; defined outcomes--seven (6%) journal articles cited the importance of having clearly stated goals with tangible outcome measures that will more likely lead to learners mastering skills; simulator validity--four (3%) journal articles provided evidence for the direct correlation of simulation validity with effective learning. While research in this field needs improvement in terms of rigor and quality, high-fidelity medical simulations are educationally effective and simulation-based education complements medical education in patient care settings.
Simulation is an approach to teaching and learning which is gaining a greater emphasis within nurse education. This has been fueled by the Nursing and Midwifery Council's (NMC) decision to identify a baseline standard for using simulation safely and its inclusion as a contributory part to practice learning [Nursing and Midwifery Council (2006a). This paper presents some of the advantages and issues for consideration in relation to its effectiveness as a teaching and learning method. Of particular concern is the limited empirical evidence to support its effect on clinical practice. Debate and further research is needed to help consolidate our knowledge and develop an evidence base for its continued use.
Nursing shortage fact Sheet: American Associaltion of Colleges of Nursing
- R Rosseter
Rosseter, R. (2011). Nursing shortage fact Sheet: American Associaltion of Colleges of Nursing.
Source: https://www.researchgate.net/publication/277324930_Pearson's_Product-Moment_Correlation_Sample_Analysis
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