Estimation
Suppose you’re at a reception for the world’s best software estimators. The room is packed, and you’re seated in the middle of the room at a table with three other estimators. All you can see as you scan the room are wall-to-wall estimators. Suddenly, the emcee steps up to the microphone and say “We need to know exactly how many people are in the room so we can order dessert. Who can give the most accurate estimate for the number of people in the room?” Bill, the estimator to your right, says “I make a hobby of estimating crowds. Based on my experience, it looks to me like we’ve got about 335 people in the room.”
The estimator sitting across the table from you, Karl, says, “This room has 11 tables across and 7 tables deep. One of my friends is a banquet planner, and she told me that they plan for 5 people per table. It looks to me like most of the tables do actually have about 5 people at them. If we multiply 11 times 7 times 5, we get 385 people. I think we should use that as our estimate.”
The estimator on your left, Lucy, says, “I noticed on the way into the room that there was an occupancy limit sign that says this room can hold 485 people. This room is pretty full. I’d say 70 to 80 percent full. If we multiply those percentages by the room limit, we get 340 to 388 people. How about if we use the average of 364 people, or maybe just simplify it to 365?”
Bill says, “We have estimates of 335, 365, and 385. It seems like the right answer must be in there somewhere. I’m comfortable with 365.”
Everyone looks at you. You say, “I need to check something. Would you excuse me for a minute?”
You return a few minutes later. “Remember how we had to have our tickets scanned before we entered the room? I noticed on my way into the room that the handheld ticket scanner had a counter. So I went back and talked to the ticket taker at the front door. She said that, according to her scanner, she has scanned 407 tickets. She also said no one has left the room so far. I think we should use 407 as our estimate. What do you say?”
The moral of this story, of course, is that you should avoid estimating what you can count. And if you can’t get a count, you should at least try to compute the estimate from a related count, like Karl did in the anecdote above. The absolute worst estimates you can produce are judgmental— estimates that are not derived from an actual count of any kind.
source: Jeff Atwood at CodingHorror.com
