A weakness of this study is the retrospective data collection of residential address history , which can be prone to recall bias . We were also limited by the numbers available for the analysis sample due to the criteria for eligibility being based on continued lifetime residence in a small geographic area . It was necessary to dichotomise OSC as to provide greater statistical power for stratified models ( i.e. to keep cell sizes to an appropriate size ) . This dichotomisation was also appropriate when this variable was used as a covariate , as previous research found that significant differences in health outcomes were only found between " professional or managerial occupations " compared to the rest ( Gale et al . , 2016 ) . We dichotomised other covariates ( e.g. smoking status ) as they were not recorded in the same way for childhood and adulthood and therefore dichotomisation was used to aid interpretation of change over time . The analysis sample was broadly similar in terms of selected characteristics compared with the full cohort at age 70 , but selection bias may have caused a higher number of participants from lower socioeconomic groups being included . The issue of selection bias was partly addressed by running the sensitivity analysis using a sample with different assumptions . For addresses within the Edinburgh area , the precision of the addresses supplied varied and therefore , for a small number of addresses that did not have a house number , the error between actual participant residence and geocoded address was larger as we took the centre of their street . In addition , by geocoding using contemporary sources , we have assumed that the street layout was the same as it is today for the majority of the addresses , with only minor changes to the street content . However we acknowledge that these small changes could have affected the precision of the park exposure estimate for the earlier time periods . Given that we were geocoding a relatively small number of addresses , we were able to employ a system to detect information lost compared with what was supplied , so that if the results were unsatisfactory they would be geocoded manually . This ensured that addresses that had changed significantly ( e.g. demolished ) would be geocoded to the same accuracy as those that were able to be geocoded with contemporary sources ( < 3 m ) . Given the complexity of the existing analysis , with multiple exposures from different time points , we were unable to include the examination of non - linearity . However , we would encourage this in future analyses , especially using the forthcoming cognitive data waves in the LBC1936 .