Validity is the best available approximation to the truth of a given proposition, inference or conclusion. Internal validity is a standard or criteria against which research results are judged.
Internal validity is the approximate truth the inference is made regarding the study that involves a causal relationship. - Trochim.
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.
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.
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.