THIS MESS
an acronym for reasons that lead to low internal validity
- Testing – subjects react on the experimental setup or task
- History – e.g. events between two measurements
- Instrument – e.g. change of the measurement tool
- Statistical regression (toward the mean) – data outliers e.g. caused by inhomogeneous test groups
- Maturation – subjects’ change between two measurements
- Experimental mortality – subjects‘ disappear
- Selection – lacking randomization of the tested sample
- Selective interaction- sequence/order effects