Between Groups & Within-Groups ANOVA How F is “constructed” Things that “influence” F –Confounding –Inflated within-condition variability Integrating “stats” & “methods”

ANOVA  ANalysis Of VAriance Variance means “variation” Sum of Squares (SS) is the most common variation index SS stands for, “Sum of squared deviations between each of a set of values and the mean of those values” SS = ∑ (value – mean) 2 So, Analysis Of Variance translates to “partitioning of SS” In order to understand something about “how ANOVA works” we need to understand how BG and WG ANOVAs partition the SS differently and how F is constructed by each.

Variance partitioning for a BG design Tx C Mean SS Total = SS Effect + SS Error Variation among all the participants – represents variation due to “treatment effects” and “individual differences” Variation between the conditions – represents variation due to “treatment effects” Variation among participants within each condition – represents “individual differences” Called “error” because we can’t account for why the folks in a condition -- who were all treated the same – have different scores.

How a BG F is constructed F = = MS effect MS error Mean Square is the SS converted to a “mean”  dividing it by “the number of things” SS effect / df effect SS error / df error df effect = k - 1 represents # conditions in design df error = ∑n - k represents # participants in study SS Total = SS Effect + SS Error

How a BG r is constructed F = = MS effect MS error r 2 = effect / (effect + error)  conceptual formula = SS effect / ( SS effect + SS error )  definitional formula = F / (F + df error )  computational forumla SS effect / df effect SS error / df error

SS total = SS effect + SS error = r 2 = SS effect / ( SS effect + SS error ) = / ( ) =.34 r 2 = F / (F + df error ) = / ( ) =.34 An Example …

A “more realistic” model of F IndDif  individual differences BG SS Total = SS Effect + SS Error F = SS effect / df effect SS error / df error ANOVA assumes there are no confounds, and that the individual differences are the only source of within-condition variability BG SS Total = SS Effect + SS confound + SS IndDif + SS wcvar SS confound  between condition variability caused by anything(s) other than the IV (confounds) SS wcvar  inflated within condition variability caused by anything(s) other than “natural population individual differences”

Imagine an educational study that compares the effects of two types of math instruction (IV) upon performance (% - DV) Participants were randomly assigned to conditons, treated, then allowed to practice (Prac) as many problems as they wanted to before taking the DV-producing test Control Grp Exper. Grp Prac DV Prac DV S S S S S S S S IV compare Ss 5&2 - 7&4 Confounding due to Prac mean prac dif between cond WG variability inflated by Prac wg corrrelation or prac & DV Individual differences compare Ss 1&3, 5&7, 2&4, or 6&8

F = SS effect / df effect SS error / df error The problem is that the F-formula will … Ignore the confounding caused by differential practice between the groups and attribute all BG variation to the type of instruction (IV)  overestimating the effect Ignore the inflation in within-condition variation caused by differential practice within the groups and attribute all WG variation to individual differences  overestimating the error As a result, the F & r values won’t properly reflect the relationship between type of math instruction and performance  we will make a statistical conclusion error ! Our inability to procedurally control variables like this will lead us to statistical models that can “statistically control” them r = F / (F + df error )

How research design impacts F  integrating stats & methods! SS Total = SS Effect +SS confound +SS IndDif +SS wcvar SS Effect  “bigger” manipulations produce larger mean difference between the conditions  larger F F = SS effect / df effect SS error / df error SS IndDif  more heterogeneous populations have larger within- condition differences  smaller F SS confound  between group differences – other than the IV -- change mean difference  changing F if the confound “augments” the IV  F will be inflated if the confound “counters” the IV  F will be underestimated SS wcvar  within-group differences – other than natural individual differences  smaller F could be “procedural”  differential treatment within-conditions could be “sampling”  obtain a sample that is “more heterogeneous than the target population”