Internal validity – means that the conclusion which comes from an experiment is valid for the respondents who participated in the experiment. For any experimental design, we always require interval validity, without which experimental results are not interpretable. Threats to Internal Validity Selection – Subjects bring with them into the investigation unique characteristics, some learned and some inherent. Examples include sex, height, weight, color, attitude, personality, motor ability, and mental ability.

If assigning subjects to comparison groups results in unequal distribution of these subject-related variables, then there is a possible threat to internal validity. History – the specific events occurring between the first and second measurement in addition to the experimental variable. Suppose that the dependent variable is measured twice for a group of subjects, once at Time A and later at Time B, and that the independent variable is introduced in the interim. Suppose also that Event A occurs between Time A and Time B.

If scores on the dependent measure differ at these two times, the discrepancy may be due to the independent variable or to Event A. Maturation – processes within the respondents operating as a function of the passage of time per se (not specific to the particular events), including growing older, growing hungrier, growing more tired/bored, and the like. Some of these changes are permanent (e. g. , biological growth), while others are temporary (e. g. , fatigue).

Suppose that the dependent variable is measured twice for a group of subjects, once at Time A and later at Time B, and that the independent variable is introduced in the interim. If scores on the dependent measure differ at these two times, the discrepancy may be due to the independent variable or to naturally occurring developmental processes. Testing – the effects of taking a test upon the scores of a second testing (like soc. desirability). Suppose that the dependent variable is recorded twice for a group of subjects, once at Time A and later at Time B, and that the independent variable is introduced in the interim.

If scores on the dependent measure differ at these two times, the discrepancy may be due to the independent variable or to the procedure involved in measuring the dependent variable at Time A. Instrumentation in which changes in the calibration of a measuring instrument or changes in the observers or scorers used may produce changes in the obtained measurements. Examples include changes in the calibration of a mechanical measuring device as well as the proficiency of a human observer or interviewer.

Suppose that the dependent variable is measured twice for a group of subjects, once at Time A and later at Time B, and that the independent variable is introduced in the interim. Suppose also that the ability of a recording device to detect instances of the target behavior improves (declines) as the experiment progresses. If scores on the dependent measure differ at these two times, the discrepancy may be due to the independent variable or to more/less sensitive recordings of the target behavior at Time B relative to at Time A.

Regression – -operating where groups have been selected on the basis of their extreme scores (e. g. , bootcamps for troubled teenagers). Take any dependent measure that is repeatedly sampled, move along it as in a time dimension, and pick a point that is the “highest/lowest so far. On the average, the next point will be lower/higher, nearer the general trend. ” Suppose that the dependent variable is measured twice for a group of subjects, once at Time A and later at Time B, and that the independent variable is introduced in the interim.

Suppose also that value observed for subjects at Time A is considerably higher/lower than would typically be the case. If scores on the dependent measure differ at these two times, it may be due to the independent variable or to a regression artifact. Mortality – or differential loss of respondents from the comparison groups. Suppose that subjects in two comparison groups differ with respect to the independent variable.

Suppose also that subjects in one group are more likely to discontinue their participation part way through an xperiment than subjects in another group and that dependent variable is measured at the end of the experiment. If scores on the dependent measure differ between those subjects remaining in the two groups, the discrepancy may be due to the independent variable or to a unique characteristic of subjects able to endure a particular condition, a subject-related variable that would be disproportionately present in each group. Selection – Biases resulting in differential selection of respondents for the comparison groups. Subject-related variables and time-related variables may interact.

Suppose that subjects in two comparison groups differ with respect to the independent variable and a subject-related variable such as age. Suppose also that the dependent variable is measured twice for each group, once at Time A and later at Time B, and that the independent variable is introduced in the interim. If the change in scores on the dependent measure from Time A to Time B differs between the two groups, this discrepancy may be due to the independent variable or to distinctive naturally occurring developmental processes for the two age categories that comprise the two comparison groups.

External Validity – asks the question of generalizability: to what extent can the experimental results be generalized? Many factors can jeopardize external validity or representativeness. The main ones are discussed as follows. When you have strong external validity, you can generalize to other people and situations with confidence. Public opinion surveys typically place considerable emphasis on defining the population of interest and drawing good samples from that population.

On the other hand, laboratory experiments often employ “convenience samples,” such as intact college classes taught by a friend. As a result, we may not know whom the subjects represent. Threats to External Validity – the reactive effect of testing, in which a pretest might increase or decrease the respondent’s sensitivity or responsiveness to the experimental variable and thus make the results obtained for a pretested population unrepresentative of the effects of the experimental variable for the unpretested universe from which the xperimental respondents were selected. The interaction effects of selection biases and the experimental variable. Reactive effects of experimental arrangements, which would preclude generalization about the effect of the experimental variable upon persons being exposed to it in no experimental settings. A threat to external validity is an explanation of how you might be wrong in making a generalization.

For instance, you conclude that the results of your study (which was done in a specific place, with certain types of people, and at a specific time) can be generalized to another context (for instance, another place, with slightly different people, at a slightly later time). There are three major threats to external validity because there are three ways you could be wrong — people, places or times. Your critics could come along, for example, and argue that the results of your study are due to the unusual type of people who were in the study.

Courtney from Study Moose

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