7 ms·
Yet another place where intuitions derived from the normal distribution about the behavior of distributions screws people. An explanation I like, from michaelo
by x0x0 12y ago
Yet another place where intuitions derived from the normal distribution about the behavior of distributions screws people.
An explanation I like, from michaelochurch:
Let's say that you have 20 tasks. Each involves rolling a 10-sided die.
If it's a 1 through 8, wait that number of minutes. If it's a 9, wait 15
minutes. If it's a 10, wait an hour.
How long is this string of tasks going to take? Summing the median time
expectancy, we get a sum 110 minutes, because the median time for a task is
5.5 minutes. The actual expected time to completion is 222 minutes, with 5+
hours not being unreasonable if one rolls a lot of 9's and 10's.
This is an obvious example where summing the median expected time for the
tasks is ridiculous, but it's exactly what people do when they compute time
estimates, even though the reality on the field is that the time-cost
distribution has a lot more weight on the right. (That is, it's more common
for a "6-month" project to take 8 months than 4. In statistics-wonk terms,
the distribution is "log-normal".)
- deleted 12y ago[deleted]
- hu_me 12y agohttps://news.ycombinator.com/item?id=3522910 https://news.ycombinator.com/item?id=3522910
- deleted 12y ago[deleted]
- idlewords 12y agoThat's a nice lucid example. Google should hire that guy!
- crasshopper 12y agoIf, as the article says, "Clients’ Focus on Low Price Is a Major Reason for Effort Overruns", then probably a simpler theoretical explanation can just be the winner's curse. No need to talk about distributions, gaussian or otherwise.
- Cacti 12y agoExponential feedback is a bitch! When I put together plans and estimates, I always take a lot of care to separate out those things which are linear and those things which exponentially impact other things within the schedule, along with the sort of inflection points. I may not know where I'm going to roll a 9 or 10, as they may crop up anywhere, but there are certainly areas where they are more possible and less possible. In a sane world, at least. Can't do much with a black swan.
- hcarvalhoalves 12y agoIn a startup I've worked on, we did estimates based on hours, but also had the "no idea" tasks. We would then make sure to alternate sprints to work on well estimated tasks (as you say, "linear" tasks) and others to work on the hard ones, without a set deadline; or split the team effort in that manner. I think that's a better strategy than making wild guesses and ultimately falling behind schedule, but at the same time maintaining cadence, which buys you power to sometimes say "hard things are hard, I don't know when it'll be ready to ship".