<P> Restriction of range, such as floor and ceiling effects or selection effects, reduce the power of the experiment, and increase the chance of a type II error . This is because correlations are attenuated (weakened) by reduced variability (see, for example, the equation for the Pearson product - moment correlation coefficient which uses score variance in its estimation). </P> <P> Greater heterogeneity of individuals participating in the study can also impact interpretations of results by increasing the variance of results or obscuring true relationships (see also sampling error ., the higher the standard deviation will be . This obscures possible interactions between the characteristics of the units and the cause - effect relationship . </P> <P> Any effect that can impact the internal validity of a research study may bias the results and impact the validity of statistical conclusions reached . These threats to internal validity include unreliability of treatment implementation (lack of standardization) or failing to control for extraneous variables . </P>

One common threat to statistical conclusion validity is