Rami,

As a project progresses, some activities will be completed early, thereby creating more total float for it’s successors. Some will be late, thereby eroding the total float for it’s successors.
If enough (non-critical) activities are late, eventually their successors will become critical and impact on the forecast completion.

Unless you look at errosion of total float and/or probability of achieving forecast completion date, you won’t notice this process happening until the forecast completion date is affected, by which time it may be too late to do anything about it.

If on the other hand you report with each update the total number of activities with, say less than 10 days total float (or one of the other methods of tracking float erosion I mentioned in my first post), you can easily see a trend developing whereby more activities are becoming near critical, and will soon delay the project if this trend is not addressed.

You can get something similar by just looking at the number of activities which are late vs baseline or previous month’s forecast, but many of these activities will have enough total float that a week’s delay will have no effect on the chances of completing on time. And some activities will be completed on time or even early, but had so little total float that their successors still present a risk of delay.