High temp (0 deg C) --- 20/20 survival

Med temp (-2 deg C) --- 6/20 survival

Low temp (-5 deg C)--- 0/20 survival

I am assuming the error is because the extremes have either no successes or no failures. Is there a legitimate way to "correct" for this problem and still do probit analysis so I can get an estimate of the lethal "dose" of temperature for 50% mortality?

One suggestions was to add one success and one failure to each of the extreme temps and then analyze using probit. The data would then look like:

High -- 21/22

Med -- 6/20

Low -- 1/22

Can anyone tell me if this is a statistically valid approach, or is there simply not much to be gained from analyzing these data?

Ken