Fig 2. Women who have used CHT in 3 to 4 years are referred to as Group A; women who have used CHT in 4 to 9 years are referred to as group B.

The arrows indicate the individual follow-up periods from first starting using CHT. In a randomized trial women are followed from time zero to the end of the arrows. In the observational studies women are only followed in the red and blue coloured part of the arrows. If the breast incidence is time-varying, then this will lead to systematic shift of diagnosis from the black part of the arrows to the coloured part of the arrows because of delay in time of diagnosis. This is leading to an over-estimation of the hazard rates in the period indicated by red and blue colours.