(8) Experimental validity

• threats to internal validity

  1. history
  2. maturation
  3. measuring instruments
  4. statistical regression
  5. experimental mortality
• history - refers to any event outside of the research study that can alter or effect subject's performance
• history - the events occuring between the first and second measurements in addition to the experimental variable which might affect the environment
• history - using randomization procedures can often minimize the risk, assuring that the events that occur in one group are also likely to occur in the other.
• maturation - refers to the natural physiological, psychological changes that take place as we age
• maturation - the process of maturing which takes place in the individual during the duration of the experiment which is not a result of specific events but of simply growing older, growing more tired, or similar changes
• measuring instruments - changes in instruments, calibration of instruments, observers, or scorers may cause changes in the measurements.
• measuring instruments - if the measurement device(s) used in your study changes during the course of the study, changes in scores may be related to the instrument rather than the independent variable
• statistical regression - refers to the tendency for subjects who score very high or very low to score more toward the mean on subsequent testing
• statistical regression - groups are chosen because of extreme scores of measurements; those scores or measurements tend to move toward the mean with repeated measurements even without an experimental variable.
• experimental mortality - the loss of subjects from comparison groups could greatly affect the comparisons because of unique characteristics of those subjects. Groups to be compared need to be the same after as before the experiment.
• external validity - refers to the generalizability of a study
• threats to external validity can result in significant results within a sample group but an inability for this to be generalized
• threats to external validity compromise our confidence in stating whether the study's result are applicable to other groups
• threats to external validity

  1. pre-testing
  2. differential selection
  3. experimental procedures
• pre-testing - individuals who went through this might be less or more sensitive to the experimental variable or might have "learned" from the pre-test making them unrepresentative of the population who didn't went through it.
• differential selection - the selection of the subjects determines how the findings can be generalized. Subjects selected from a small group or one with particular characteristics would limit generalizability. Randomly chosen subjects from the entire population could be generalized to the entire population.
• experimental procedures - have a certain amount of effect on the subjects in the experimental settings. Generalization to persons not in the this settings may be precluded.
• if the study shows a high degree of internal validity, then we can conclude that we have strong evidence of causality
• if a study has low internal validity, then we can conclude that the study has little or no evidence of causality