I need help understanding this paragraph! PLEASE!?

I read it as the statistics were false because they stated that the survival rate increased but in actuality they couldn't track if they were all alive or not because they moved out of state and what not. Instead of beginning a study and following through for 5 years they started in 1941 and just looked at charts and such back the past 5 years so they actually don't know what happened with the studied patients because they didn't follow through. That's how I read it.