The main limitations of the study are that the sample was a selected one and that, because of the conditions of sample selection (anonymous diabetes register, registration of the cases a relatively long time ago), little information on nonparticipating patients was available.
A more ample evaluation of sample selection bias was not feasible because age and sex were the only available data on the Tuscan population.
As for Fig. 9, this comes from the effects of sample selection and our sample is inclined to more luminous galaxies because of the IR-selection.
Because of the sample selection strategy, this study is not addressed to evaluate the effect of very recent genetic flow from outside populations - essentially from the Italian mainland - that, according to demographic data, occurred mainly during the last sixty years.
Although the OLS estimates suggested negative coefficients for CPI, the CPI coefficient could be negatively biased in the OLS regression because of the sample selection problem as stated in Section 4. Indeed, if the FE method is employed, the effect of CPI on bonuses became significantly positive.
This is probably because of differences in sample selection; while Kahn (2010) dropped 1957 cohorts, I dropped those who disappeared from the data by age 30.
The two groups were not statistically different in age, gender, or race/ethnicity, probably because of the matching sample selection procedure.
The higher household income level of the sample in comparison with the average household income of Mekong Delta residents (about 5.2 million per month in 2010b) does not show a problem in terms of a sample selection bias because the difference could be explained by the inclusion of rural households who earn lower income than urban residents in the population average.
It is usually difficult to compare results of surveillance studies because of differences in the sample selection and the criteria of mastitis.
