Guessing and Estimating – and Knowing the Difference

Here’s a thought to ponder: when is guessing estimating and when is estimating guessing? It’s a conundrum addressed well by Glenn B. Alleman in his excellent blog, “Herding Squirrels.”

As I’ve openly admitted in the past, I first latched on to Glenn’s blog simply because of the name. In my experience as a manager, I’ve often identified with the sentiment that I was herding squirrels.

Yet, Alleman tackles the excellent subject of guessing and estimating in a post on his blog called, “Why guessing is not estimating and why estimating is not guessing.” He comes to this belief quickly, “What’s the difference between estimate and guess? The distinction between the two words is one of the degrees of care taken in arriving at a conclusion.”

As he observes, estimating is not the same as guessing. For example, he says it’s just not mathematically possible. It’s also not a wise business proposition either.

To understand both, it helps to know the root meaning of the words. (Somewhere my high school English teacher is smiling down on me for discussing “roots” in an article.) As Alleman points out, “The word ‘estimate’ is derived from the Latin word aestimare, meaning to value.” He goes on to point out, “To estimate means to judge the extent, nature, or value of something.”

Alleman doesn’t get into the root meaning of guess, so I will as much as I can. Its roots are in Middle English, with uncertain origins. It may be based on the Dutch for gissen, which is related to the phrase, “To get.” In Alleman’s view, it’s probably related to the phrase, “To get lucky.” This is how he defines the word, “To guess is to believe or suppose, to form an opinion based on little or no evidence, or to be correct by chance or conjecture.”

So, in his view, what’s the difference between the two? He says, “A guess is a casual, perhaps spontaneous conclusion. An estimate is based on intentional thought processes supported by data.” Obviously, there is a huge world of difference between a guess and an estimate. It’s the difference between uninformed conclusions and informed conclusions.

Shim Marom, writing at his blog, quantmleap, says an estimate “is a quantitative approximation.” He then adds, “A guesstimate is a quantitative approximation NOT based on previously observed data – and is rather based on gut-feel and a guess.”

Of course, you can’t discuss estimates and guesstimates without discussing “WAGs” or as the less couth might say, “wild ass guesses.” Margaret Rouse, writing at, has this to say, it “is an off-the-cuff estimate for something, such as the completion date of a project, when there is insufficient data available to support a more informed estimation.”

Where do WAGs fit into all of this? Apparently there are methods for turning WAGs into something called SWAGs, or scientific wild ass guesses. At, Bob Zimmerman writes, “Whenever someone asks you for high-level-estimate and you can’t really do proper research, you will need to make assumptions.  The key is to expose these assumptions with your estimate and to demonstrate how changes in assumptions might impact your estimate.”

Zimmerman offers this framework for creating SWAGs:

  • Estimates should include a range. Then provide assumptions for those numbers. “It sounds simple, but the difference is pretty significant. Always start out by thinking: What is the lowest reasonable number your estimate might be,” he says.
  • Then be honest about how long the task is going to take to complete. “When you give this estimate, you also share what assumptions need to be true for you stay within this estimate,” he says.

Zimmerman offers these bullet points to turn a WAG into a SWAG:

  • Provide the shortest or lowest estimate that is reasonable
  • Explain why it isn’t reasonable to be shorter/lower
  • Explain how large it is possible to go
  • Outline what are the assumptions that would make it go that large
  • What are your assumptions that could cause it to go larger

Finish your estimate with a viable, in between reasonable “target” in-between that you believe is reasonable to start with. Explain how and when you will be able to validate assumptions and firm up your estimate.

Who knew that so much could go into guessing and estimating? Share your thoughts below in the comments section. How do you address guessing and estimating?

ProjectManagers Team

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