# Power Analysis for Correlation & Multiple Regression

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1 Power Analysis for Correlation & Multiple Regression Sample Size & multiple regression Subject-to-variable ratios Stability of correlation values Useful types of power analyses Simple correlations Full multiple regression Considering Stability & Power Sample size for a study Sample Size & Multiple Regression The general admonition that larger samples are better has considerable merit, but limited utility R² will always be 1.00 if k = N-1 (it s a math thing) R² will usually be too large if the sample size is too small (same principle but operating on a lesser scale) R² will always be larger on the modeling sample than on any replication using the same regression weights R² & b-values will replicate better or poorer, depending upon the stability of the correlation matrix values R² & b-values of all predictors may vary with poor stability of any portion of the correlation matrix (any subset of predictors) F- & t-test p-values will vary with the stability & power of the sample size both modeling and replication samples Subject-to-Variable Ratio How many participants should we have for a given number of predictors? -- usually refers to the full model The subject/variable ratio has been an attempt to ensure that the sample is large enough to minimize parameter inflation and improve replicability. Here are some common admonitions.. 20 participants per predictor a minimum of 100 participants, plus 10 per predictor 10 participants per predictor 200 participants for up to k=10 predictors and 300 if k> participants per predictor a minimum of 2000 participants, for each 5 predictors

2 As is often the case, different rules of thumb have grown out of different research traditions, for example chemistry, which works with very reliable measures and stable populations, calls for very small s/v ratios biology, also working largely with real measurements (length, weight, behavioral counts) often calls for small s/v ratios economics, fairly stable measures and very large (cheap) databases often calls for huge s/v ratios education, often under considerable legal and political scrutiny, (data vary in quality) often calls for fairly large s/v ratios psychology, with self-report measures of limited quality, but costly data-collection procedures, often shaves the s/v ratio a bit Problems with Subject-to-variable ratio #1 neither n, N nor N/k is used to compute R² or b-values R² & b/-values are computed from the correlation matrix N is used to compute the significance test of the R² & each b-weight #2 Statistical Power Analyses involves more than N & k We know from even rudimentary treatments of statistical power analysis that there are four attributes of a statistical test that are inextricably intertwined for the purposes of NHST acceptable Type I error rate (chance of a false alarm ) acceptable Type II error rate (chance of a miss ) size of the effect being tested for sample size We will forsake the subjects-to-variables ratio for more formal power analyses & also consider the stability of parameter estimates (especially when we expect large effect sizes). NHST Power vs. Parameter estimate stability NHST power what s the chances of rejecting a false null vs. making a Type II error? Statistical power is based on size of the effect involved ( larger effects are easier to find ) amount of power (probability of rejecting H0: if effect size is as expected or larger) Stability how much error is there in the sample-based estimate of a parameter (correlation, regression weight, etc.)? Stability is based on quality of the sample (sampling process & attrition) sample size Std of r = 1 / (N-3), so N=50 r +/-.146 N=100 r +/-.101 N=200 r +/-.07 N=300 r +/-.058 N=500 r +/-.045 N=1000 r +/-.031

3 The power table only tells us the sample size we need to reject H0: r=0!! It does not tell us the sample size we need to have a good estimate of the population r!!!!! Partial Power Table (taken & extrapolated from Friedman, 1982) r power Sufficient power but poor stability How can a sample have sufficient power but poor stability? Notice it happens for large effect sizes!! e.g., For a population with r =.30 & a sample of 100 Poor stability of r estimate +/- 1 std is Large enough to reject H0: that r = 0 power almost.90 Power Analysis for Simple Correlation Post hoc I found r (22) =.30, p >.05, what s the chance I made a Type II error?? A priori N = 24 Power =.30 Chance Type II error.70 #1 I expect my correlation will be about.25, & want power =.90 sample size should be = #2 Expect correlations of.30,.45, and.20 from my three predictors & want power =.80 sample size should be = , based on lowest r =.20 Power Analysis for Multiple regression Power analysis for multiple regression is about the same as for simple regression, we decide on values for some parameters and then we consult a table Remember the F-test of H0: R² = 0?? R² / k R² N-k-1 F = = * R² / N - k R² k Which corresponds to: significance test = effect size * sample size So, our power analysis will be based not on R² per se, but on the power of the F-test of the H0: R² = 0

4 Using the power tables (post hoc) for multiple regression (single model) requires that we have four values: a = the p-value we want to use (usually.05) u = df associated with the model ( we ve used k ) v = df associated with F-test error term (N - u - 1) f² = (effect size estimate) = R² / (1 - R²) λ = f² * ( u + v + 1) This is the basis for determining power E.g., N = 96, and 5 predictors, R² =.10 was found a =.05 u = 5 v = = 90 f² =.1 / (1 -.1) =.1111 λ =.1111 * ( ) = 10.6 Go to table -- a =.05, & u = 5 λ = v = Another N = 48, and 6 predictors, R² =.20 (p <.05) a =.05 u = 6 v = f² =.2 / (1 -.2) =.25 λ =.25 * ( ) = 12 Go to table -- a =.05 & u = 6 λ = 12 v = 20 power is about This sort of post hoc power analysis is, as before, especially helpful when the H0: has been retained -- to determine whether the result is likely to have been a Type II error. Remember that one has to decide how small of an effect is meaningful, and adjust the sample size to that decision power is around a priori power analyses for multiple regression are complicated by... Use ofλ (combo of effect & sample size) rather than R² (just the effect size) in the table. This means that sample size enters into the process TWICE when computing λ = f² * ( u + v + 1) when picking the v row to use v = N - u - 1 So, so the λ of an analysis reflects the combination of the effect size and sample size, which then has differential power depending (again) upon sample size (v). E.g.#1, R² =.20 f² =.2 / (1-.2) =.25 N = 50 λ =.25 * ( 50) = 12.5 with u = 10, and v = N = power is about.50 E.g.#2, R² =.40 f² =.4 / (1 -.4) =.67 N = 19 λ =.67 * (19) = 12.5 with u = 10, and v = = 8 -- power is about.22 So, for a priori analyses, we need the sample size estimate to compute the λ to use to look up the sample size estimate we need for a given level of statistical power????

5 Perhaps the easiest way to do a priori sample size estimation is to play the what if game... I expect that my 4-predictor model will account for about 12% of the variance in the criterion -- what sample size should I use??? a =.05 u = 4 f² = R² / (1 - R²) =.12 / (1 -.12) =.136 what if.. N = 25 N = 65 N = 125 v = (N - u - 1) = λ = f² * ( u + v + 1) = Using the table power = about.21 about.62 about.915 If we were looking for power of.80, we might then try N = 95 so v = 90, λ = 12.2, power = about.77 (I d go with N = ) Putting Stability & Power together to determine the sample size 1. Start with stability remember Std of r = 1 / (N-3), so N=50 r +/-.146 N=100 r +/-.101 N=200 r +/-.07 N=300 r +/-.058 N=500 r +/-.045 N=1000 r +/-.031 suggesting that is a good guess for most analyses (but more is better). 2. Then for the specific analysis, do the power analysis For the expected r/r² & desired power, what is the required sample size? 3. Use the larger of the stability & power estimates! An example. We expect a correlation of.60, and want only a 10% risk of a Type II error if that is the population correlation Looking at the power table for r =.60 and power =.90.. the suggested sample size is 21 N = 21, means the std of the correlation estimates (if we took multiple samples from the target population is 1 / (21-3) =.35 With N = 21 we ve a 90% chance of getting a correlation large enough to reject the Null on average, our estimate of the population correlation will be wrong my.35. We d certainly interpret a.25 and a.95 differently In this case we d go with the estimate, in order to have sufficient stability we ll have lots of power!

6 Another example. We expect a correlation of.10, and want only a 20% risk of a Type II error if that is the population correlation Considering stability let s say we decide to go with 300 Looking at the power table for r =.10 and power =.80.. the suggested sample size is 781 With N = 300, we d only have power of about.40 60% chance of a Type II error. In this case we d go with the 781 estimate (if we can afford it), in order to have sufficient power we ll have great stability of +/-.036! Considering the sample size for the Study Really a simple process, but sometimes the answer is daunting! First: For each analysis (r or R²) perform the power analysis consider the suggestion & resulting stability pick the larger value as the N estimate for that analysis Then: Looking at the set of N estimates for all the analyses The largest estimate is the best bet for the study This means we will base our study sample size on the sample size required for the least powerful significance test! Usually this is the smallest simple correlation or a small R² with a large number of predictors.

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