An experiment is said to possess internal validity in so far as it controls the effects of all non-manipulated variables so that the differences among experimental groups on the dependent variable can be treated as valid effects of the manipulations. An experiment is internally valid so far as the observed differences among the treatment groups are valid effects of the manipulations. In other words, internal validity removes causal uncertainty.
Several factors affect the internal validity of the experiments.
Selection bias The participants in the experimental groups may be different from those of other groups. The effects then could be attributed to the differences in the groups, rather than the manipulation of the causal variable.
Treatment effects The very fact that it is an experiment tend to affect the dependent variable. The subjects or participants tend to fall in line with the experimental thinking e.g. people tend to choose a cover design that they think is desirable. Participants may also tend to look more favourably.
Testing effects When a dependent variable is being measured after manipulation, the testing procedure tends to affect the results. It is because the pre-test tends to make us fall in line with the desired outcome at the time of post-test.
History effects These happen when an outside event affects the dependent variable during one experiment, e.g. competitive action.
Maturation Effects In this there is some change in the participants which is unrelated to the manipulation. It is this change that affects the dependent variable.
Mortality Effects Some participants may not last till the end of the experiment. Desertions do affect the observed effects.
The following table summarises the above discussion.
Threats to Internal Validity
Threat Reason
Selection bras O Differences in the participants.
Treatment Effects O The very fact that it is an experiment
O The desire to look favourable
O The desire to fall in line with the researcher’s thinking.
Testing Effects O Interaction with the measurement procedur
History Effects O External environment changes
Maturation effects O Internal changes in the participants.
Mortality Effects O Some participants desert the experiment
External Validity
An experiment is externally valid so far as the effects occuring are closer to those in an actual market situation. It is also called generalizability as the effects are generalized to the market place. In the book cover experiment, the two designs sent to different booksellers are high on external validity. Test marketing is also high on enternal validity. When book covers are compared in office by the employees of the publishing house, the experiment is low on external validity. Here book covers are not tested under realistic market conditions. The employees are not representative of buyers of the books.
Selection bias The participants in the experimental groups may be different from those of other groups. The effects then could be attributed to the differences in the groups, rather than the manipulation of the causal variable.
Treatment effects The very fact that it is an experiment tend to affect the dependent variable. The subjects or participants tend to fall in the line with the experimental thinking, e.g. people tend to choose a cover design that they think is desirable. Participants may also try to look more favourably.
Testing effects When a dependent variable is being measured after manipulation, the testing procedure tends to affect the results. It is because the pre-test is in line with the desired outcome at the time of post-test.
History effects These happen when an outside event affects the dependent variable during one experiment, e.g. competitive action.
Maturation effects In this there is some change in the participants which is unrelated to one manipulation. It is this change that affects the dependent variable.
Marketing effects Some participants may not last till the end of the experiment. Desertions do affect the observed effects.