Simple Correlation RH:correlation questions/hypotheses and related statistical analyses– simple correlation– differences between correlations in a group– difference between correlations in different groupsSimple correlation Questions…1. “Are two variables correlated, within a specific group?”• H0: There is no linear relationship (correlation) between the practice and performance in the population represented by the sample•H0: rperf, prac= 0.0• get used to working with both r (the correlation between the 2 vars ) and r2 (the “variance shared between the 2 vars)”Performing the significance test• software will usually provide an exact p-value (use p < .05)• a general formula is …r² N = sample sizeF = ------------------------(1 - r²) / (N - 2) Find F-critical using df = 1 & N-2Kinds of Simple correlation questions, cont.2. “Are two correlations different, within a single group?”Common versions of this question include…•“Which is better correlated with performance, practice or prior skill?”• “Which is better correlated with practice, performance or confidence?”H0: The correlation between practice and performance is the same as between prior skill and performance, in the population represented by the sample.H0: rperf, prac= rperf, pskillThis is a very important test to understand -- it turns out that it is used with several different tests of bivariate and multivariate research hypotheses.Please note that you will need four pieces of information to apply those formulas:ry,x1ry,x2 rx1,x2 (corr of variables being compared) NHotelling’s t-test and Steiger’s Z-test are used to answer this question?? Nasty formulas, but both are in the FZT computatorKinds of Simple correlation questions, cont.3. “Is a correlation different across two groups?”H0: The correlation between practice and performance is the same for novice and for experienced participantsH0: rperf, pracfor novices = rperf, practfor experiencedThis test is very important because it is used to ask if a correlation value “generalizes” across groups or populations -- an important question for both theory and application.Identify the kinds of “correlation question” for each …Is age a better predictor of social skills for boys than girls?Is age a better predictor of social skills than SES?Does age predict social skills?Does IQ predict school performance?Does IQ predict school performance better than SES?Does SES predict IQ better for children or adults?1st moment of caution when comparing correlations!You have to decide if you are going to compare ….the “correlations” of the two predictors (including sign + or -)or the “strength”, r2, |r|, or “predictive utility” of the two predictors (ignoring the sign)For example:r(98) = .35 for # correct and confidence ratingsr(98) = -.25 for # correct and time to complete the task(r = -.45 for confidence and time to complete)Comparing .35 & -.25 yields Z = 3.55, p < .01 Æ different rComparing .35 & .25 yields Z = .63, p > .05 Æ same r2Notice that these questions are equivalent if the signs of the two correlations are the same!2nd moment of caution when comparing correlations!Don’t confuse asking…• if each variable is significantly correlated with the criterion vs. • if the variables are differentially correlated with the criterionExample…r(28) = .37, p < .05 for # correct and time to complete the taskr(28) = .33, p > .05 for confidence and time to complete the taskAlthough # correct is significantly correlated with time to complete the task and confidence is not significantly correlated with time to complete, it is a different question to ask if the two correlations are significantly different!Said differently Æ There may not be a significant difference between one significant correlation and another non-significant correlation.An important variation on this theme…While it is most common to apply these models to ask which of two variables is the better predictor of a given criterion…… it is possible to apply them to ask for which criterion a givenvariable is the better predictor.Often we collect multiple variables that are considered “outcome”or criterion variables. If so, when we talk about how good a predictor is, it is important to know if the effectiveness of the predictor depends upon the criterion we are using.Remember – like in the other applications of these models …• it is different to say that a predictor is correlated with one criterion and not correlated with another, than to say it is differentially correlated with the two!• it is different to ask if two correlations are significantly different than to ask if the two r2or |r| are