Just 16 pages into Fred Brooks’ essential reading, The Mythical Man Month, Fred gets down to business to explain that:
Cost does indeed vary as the product of the number of men and the number of months. Progress does not.
The classical way of thinking is that you can exchange one for the other… if you want something to take less months, you add more people. This is wrong. The marginal economics to shave off one month change each time we add a person. If you have a ten-month task being undertaken by one person, the marginal economics look like this (based on a perfectly partitionable task)…
- 1 person +1: -5 months
- 2 people +1: -2 months
- 3 people +1: -1 month
- 4 people +1: -a few days
So unless the cost of delay is much more than the cost of the additional person, it soon makes little sense to add “the next one”.
And if the task has complex interrelationships, adding the next person may actually increase the number of months. This is a death spiral, because this often results in management reacting by adding more people, when removing people would do more good.
The much repeated phrase, from the same book, is:
Everyone knows it takes a woman nine months to have a baby. But you Americans think if you get nine women pregnant, you can have a baby in a month
But I think this over-simplifies the charts Fred uses to describe different kinds of task and what the payoff of adding people is (at the start of the work APLPMIL). I prefer to think of growing plants…
If you want to grow an average plant, you need to determine the right amount of water. Both too little and too much water will cause you problems. You will need to find out the right kind of amount to get the best result. If you simply keep adding water to try and produce a flower sooner, you will find that some additional water over the right amount makes no difference – and that adding even more will actually have the opposite effect.
So you need to learn when to increase the water, and when to decrease the water… and when to improve the quality of the water.