Validity is defined in an experiment on how correct the results are of an experiment. Internal validity measures this from within the experiment; ensuring that the variables intended to be manipulated in the experiment are the only ones that were manipulated (Myers, & Hansen, 2012). External validity measures the extent to which the results of an experiment can be generalized to other populations outside the experiment. Ways to increase external validity include aggregation, multivariate designs, non-reactive measurements, field experiments, and natural observation. Aggregation is gathering data and averaging it in many ways (Myers, & Hansen, 2012). There are four types of aggregation: over subjects, over stimuli or situations, over trials or occasions, and over measures. The second way is to use multivariate designs; multivariate designs allow researchers to analyze multiple dependent variables at a time. This allows researchers to look at a combination of variables that will generalize better with populations outside the experiment.