As you could see from the examples presented, no single data collection covers all your population of interest.

Further, not all groups are easy to access (e.g. those only on mobile phones, the homeless, the reluctant elderly, the youngster who is never at home to be interviewed).

If intending to compare data across states or other geographic areas to obtain prevalence, you may find that other projects have different definitions of “elderly”, “teenagers” and “women of reproductive age”.

Select an agreed standard definition used in the literature by others so you can compare your parameters with other studies. This may not always be possible.

Finally, sometimes less technical and more practical issues sucha as timeliness and budget restrictions may be the reason why you cannot undertake a perfectly rigorous study. At times, political pressure may lead you to settle for the second best option. The important issue is that you acknowledge these limitations when you are writing the report.