Researchers and publishers have called for improved reporting of external validity items and for testing of existing tools designed to assess reporting of items relevant to external validity. Few tools are available and most of this work has been done within the field of health promotion.
We tested a tool assessing reporting of external validity items which was developed by Green & Glasgow on 39 studies assessing the health impacts of housing improvement. The tool was adapted to the topic area and criteria were developed to define the level of reporting, e.g. “some extent”. Each study was assessed by two reviewers.
The tool was applicable to the studies but some items required considerable editing to facilitate agreement between the two reviewers. Levels of reporting of the 17 external validity items were low (mean 6). The most commonly reported items related to outcomes. Details of the intervention were poorly reported. Study characteristics were not associated with variation in reporting.
The Green & Glasgow tool was useful to assess reporting of external validity items but required tailoring to the topic area. In some public health evaluations the hypothesised impact is dependent on the intervention effecting change, e.g. improving socio-economic conditions. In such studies data confirming the function of the intervention may be as important as details of the components and implementation of the intervention.