Given the data, they concluded that social scientists could not possibly have picked a worse population [Westerners] from which to draw broad generalizations. Researchers had been doing the equivalent of studying penguins while believing that they were learning insights applicable to all birds. We Aren’t the World Pacific Standard, Feb 25, 2013.

Very cool.  What we are often so sure of scientifically about human nature doesn’t always work elsewhere in the world.  Our fear of suggesting that  people are different and hence treating them like they are all the same, blind our scientific inquiry to the fact that people are … different.

However, in small isolated situations, such as a project, predicting behavior and performance based upon recent past results, will work very well.

See for example Is Your Project Schedule Really Driven By Your Requirements?

What assumptions are you using that upon reflection might be inappropriate and a reason for why things are not working out as expected?