Thirdly, the domain restriction to the electoral roll, which enhanced the internal validity, possibly reduced external validity.
A limitation is that the external validity is reduced to facilities with similar socioeconomic and demographic characteristics and healthcare systems with comparable standards.
We still face the unresolved question of validity: We reduced the number of target variables but still we do not know what we are measuring.
Thus, as Tarski notes, if every expression of a language counted as a logical constant, logical validity would reduce to material truth preservation (or, on later versions of Tarski's definition, to material truth preservation on every nonempty domain) (1983, 419).
Internal validity biases reduce confidence in the results.
To improve the internal validity by reducing bias, the following actions are taken.
Attrition has been recognized as a factor that threatens internal validity and reduces statistical power in a study.
However, the exclusion of these clients limits the external validity by reducing the representativeness of the study to all clients of psychiatric hospitals.
Construct validity: To reduce the number of statements and to emphasize clarity an exploratory factor analysis was carried out using principal component analysis (PCA) with Oblimin rotation [ 29].
Efficacy studies aim to produce results with high internal validity by reducing both bias (e.g. using blinding) and variation (e.g. narrow eligibility criteria and a standardized treatment protocol) to increase the likelihood of detecting the hypothesized difference.
