- The exposure information will be provided by the study participants or their next of kin (if the outcome is mortality). Recall bias may very possible occur when the information is not "recorded" and is recalled by the individuals. 

- In the design each individual serves as his/her own control and this can eliminate confounding by subject characteristics. But if there is certain characteristics which change over time in the individual, self control can’t adjust for this. For example, an individual usually drinking coffee after exercise represents a within-individual confounding, since it is difficult to distinguish whether coffee consumption or exercise causes the outcome of interest. If this happens, one can stratify the data or use conditional logistic regression to adjust for the confounding.