Internal validity refers to the validity of the study itself. In a study that has high internal validity, the outcomes can confidently be said to directly result from the study's manipulation. Validity is the best available approximation to the truth of a given proposition, inference or conclusion. If the results can be seen as produced by the way the experiment or survey was conducted then the results are internally invalid. Internal validity refers to the extent to which we can accurately state that the independent variable produced the observed effect. Internal validity is a criteria against which research results are judged. Problems related to selection, bias, maturation and attrition, among others, can negatively affect a design's internal validity, according to clinical psychologist Steven Taylor and clinical researcher Gordon J. G. Asmundson in their chapter in the “Handbook of Research Methods in Abnormal and Clinical Psychology.”
To be internally valid the results of an experiment or of a survey are considered to be accurate indications of the manipulation of an independent variable in the case of an experiment, or of the attitudes or knowledge of respondents in the case of a survey. External Validity refers to the accuracy of scientific results when generalized beyond the laboratory or survey situation to the real world.
Something internal to the research process produced the results, so researchers are no longer measuring what they claim to be measuring. Placebos and double blind procedures in experiments are used to enhance internal validity. Threats to internal validity are categorized into three groups depending on the nature of the research, and how it is designed. These include a single group threat, multiple group threat and social threat to internal validity.
A single group threat to internal validity occurs when an experiment or treatment involves a single group. That is, researchers or experimenters are not using a comparison group in their causal relationship study. The multiple group threat to internal validity refereed to the conditions in which the two groups are not comparable before the study. The social threat to internal validity refers to the social pressures in the research context that can lead to post test differences that are not directly caused by the treatment itself. - Trochim.