While the credibility at the threshold of these exercises is typically not an issue, the external validity for regions far from the cutoff is a major drawback, especially for exercises that aim to inform policy.
However, the external validity for fractures in general appeared to be satisfactory.
The recruitment procedure may decrease the external validity for patients seen in primary care as students are recruited through schools instead of through GPs.
Therefore, a certain selection bias cannot be excluded and the external validity for other high caries populations, or age groups, is unclear.
In this cohort profile, we aim to describe our study protocol, present its first results and ongoing data collection, and discuss the methodological issue of potential participation bias at study entry, which is one of the concerns for the external validity of cohort studies, for example, it is often assumed that the more healthy group is over-represented.
Further support for the external validity of the job done for most of working life measure comes from comparison with data from a cross-sectional study of slightly younger adults in North Staffordshire, in which 25% of adults aged 50 and over stated that the job they had held for the longest time was also their current job [ 40].
This lack of representativeness has clear implications for the external validity of trial findings, as well as for the common tendency to extrapolate trial data for commissioning purposes (National Institute for Clinical Excellence, 2003; Murthy et al, 2004; Cronin et al, 2005; Vickers, 2008; Estey, 2009; Friedberg et al, 2009).
Also, in the absence of local data, hospitalisation rates were taken from the international IMpact trial, further limiting the external validity of the results for any one country.
On the other hand, the cultural similarities between Iran and some other Eastern countries can increase the external validity of our model for policy makers in such countries.
The favourable comparison of the overall proportion of glass, ceramics or pottery longest jobs in the NorStOP cohort in 2002 with the proportion of current or most recent pottery related jobs in the 1981 Census data for Stoke-on-Trent support the external validity of the job done for most of working life measure in a postal survey of older adults.
As for the external validity of our study, GPs in the Netherlands, frequently use a typically Dutch expression (pluis/niet- pluis) to indicate gut feelings (Stolper et al. 2009a) and we assume that familiarity with this concept facilitates discussions about gut feelings among Dutch GPs.
