Naturally occurring cell death complicates the analysis of cell lineage studies by making the surviving members of a clone appear more closely related than they actually are. Here we ask how much normal cell death occurs during rat retinal development, and whether that amount of death is sufficient to confuse the analysis of cell lineage relationships. We measure total cell death in the retina by combining relative counts of dead cells with absolute measurements of total cell loss. For most cell types, but not rods, we find that half of the cells generated die during normal retinal development. We use a computer model to quantify the effects of different amounts of cell death in a simulated lineage study. The simulation indicates that 50% cell death means that clonal variability analysed after the cell death period is not necessarily a good indicator of how much variability actually occurs in the underlying lineage.