Single-Group Threats to Internal Validity

The Single Group Case Two designs:

The Single Group Case AdministerprogramMeasureoutcomesXO Two designs:

The Single Group Case AdministerprogramMeasureoutcomesXO Two designs: AdministerprogramMeasureoutcomesXO Measurebaseline O

The Single Group Case AdministerprogramMeasureoutcomesXO Two designs: AdministerprogramMeasureoutcomesXO Measurebaseline O Alternativeexplanations Alternativeexplanations Alternativeexplanations

ExampleExample l Compensatory education in math for 1st graders l Pre-post single group design l Measures (O) are standardized achievement tests (at start of grade 1 and start of grade 2; Forms A & B)

History Threat l Any other event that occurs between pretest and posttest l For example, kids pick up math concepts watching Sesame Street ProgramPosttest XO Pretest O

Maturation Threat l Normal growth between pretest and posttest. l They would have learned these concepts anyway, even without program. ProgramPosttest XO Pretest O

Testing Threat l The effect on the posttest of taking the pretest l May have “primed” the kids or they may have learned from the test, not the program ProgramPosttest XO Pretest O

Instrumentation Threat l Any change in the test from pretest and posttest l For example, change due to different forms of test, not to program ProgramPosttest XO Pretest O

Mortality Threat l Nonrandom dropout between pretest and posttest l For example, kids “challenged” out of program by parents or teachers ProgramPosttest XO Pretest O

Regression Threat l Group is a nonrandom subgroup of population. l For example, mostly low-math kids will appear to improve because of regression to the mean. ProgramPosttest XO Pretest O

Regression Threat ProgramPosttest XO Pretest O