Tag Archives: fiction

Estimable Stories in the INVEST Model

Estimable stories can be estimated: some judgment made about their size, cost, or time to deliver. (We might wish for the term estimatable, but it’s not in my dictionary, and I’m not fond enough of estimating to coin it.)

To be estimable, stories have to be understood well enough, and be stable enough, that we can put useful bounds on our guesses.

A note of caution: Estimability is the most-abused aspect of INVEST (that is, the most energy spent for the least value). If I could re-pick, we’d have “E = External”; see the discussion under “V for Valuable”.

Why We Like Estimates

Why do we want estimates? Usually it’s for planning, to give us an idea of the cost and/or time to do some work. Estimates help us make decisions about cost versus value.

When my car gets worked on, I want to know if it’s going to cost me $15 or $10K, because I’ll act differently depending on the cost. I might use these guidelines:

  • < $50: just do it
  • $50-$300: get the work done but whine to my friends later
  • $300-$5000: get another opinion; explore options; defer if possible
  • $5000+: go car shopping

Life often demands some level of estimation, but don’t ignore delivery or value to focus too much on cost alone.

We’ll go through facts and factors affecting estimates; at the end I’ll argue for as light an estimation approach as possible.

Face Reality: An Estimate is a Guess

If a story were already completed, the cost, time taken, etc. would be (could be?) known quantities.

We’d really like to know those values in advance, to help us in planning, staffing, etc.

Since we can’t know, we mix analysis and intuition to create a guess, which could be a single number, a range, or a probability distribution. (It doesn’t matter whether it’s points or days, Fibonacci or t-shirt sizes, etc.)

When we decide how accurate our estimates must be, we’re making an economic tradeoff since it costs more to create estimates with tighter error bounds.

How Are Estimates Made?

There are several approaches, often used in combination:

  • Expert Opinion AKA Gut Feel AKA SWAG: Ask someone to make a judgment, taking into account everything they know and believe. Every estimation method boils down to this at some point.
  • Analogy: Estimate based on something with similar characteristics. (“Last time, a new report took 2 days; this one has similar complexity, so let’s say 2 days.”)
  • Decomposition AKA Divide and Conquer AKA Disaggregation: Break the item into smaller parts, and estimate the cost of each part — plus the oft-forgotten cost of re-combining the parts.
  • Formula: Apply a formula to some attributes of the problem, solution, or situation. (Examples: Function Points, COCOMO.)
    • formulas’ parameters require tuning based on historical data (which may not exist)
    • formulas require judgment about which formulas apply
    • formulas tend to presume the problem or solution is well-enough understood to assess the concrete parts
  • Work Sample: Implement a subset of the system, and base estimates on that experience. Iterative and incremental approaches provide this ongoing opportunity.
  • Buffer AKA Fudge Factor: Multiply (and/or add to) an estimate to account for unknowns, excessive optimism, forgotten work, overheads, or intangible factors. For example: “Add 20%”, “Multiply by 3”, or “Add 2 extra months at the end”.

Why Is It Hard to Estimate?

Stories are difficult to estimate because of the unknowns. After all, the whole process is an attempt to derive a “known” (cost, time, …) from something unknowable (“exactly what will the future bring?”).

Software development has so many unknowns:

  • The Domain: When we don’t know the domain, it’s easier to have misunderstandings with our customer, and it can be harder to have deep insights into better solutions.
  • Level of Innovation: We may be operating in a domain where we need to do things we have never done before; perhaps nobody has.
  • The Details of a Story: We often want to estimate a story before it is fully understood; we may have to predict the effects of complicated business rules and constraints that aren’t yet articulated or even anticipated.
  • The Relationship to Other Stories: Some stories can be easier or harder depending on the other stories that will be implemented.
  • The Team: Even if we have the same people as the last project, and the team stays stable throughout the project, people change over time. It’s even harder with a new team.
  • Technology: We may know some of the technology we’ll use in a large project, but it’s rare to know it all up-front. Thus our estimates have to account for learning time.
  • The Approach to the Solution: We may not yet know how we intend to solve the problem.
  • The Relationship to Existing Code: We may not know whether we’ll be working in a habitable section of the existing solution.
  • The Rate of Change: We may need to estimate not just “What is the story now?” but also “What will it be by the end?”
  • Dysfunctional Games: In some environments, estimates are valued mostly as a tool for political power-plays; objective estimates may have little use. (There’s plenty to say about estimates vs. commitments, schedule chicken, and many other abuses but I’ll save that for another time.)
  • Overhead: External factors affect estimates. If we multi-task or get pulled off to side projects, things will take longer.

Sitting in a planning meeting for a day or a week and ginning up a feeling of commitment won’t overcome these challenges.

Flaws In Estimating

We tend to speak as if estimates are concrete and passive: “Given this story, what is the estimate?”

But it’s not that simple:

  • N for Negotiable” suggests that flexibility in stories is beneficial: flexible stories help us find bargains with the most value for their cost. But the more variation you allow, the harder it is to estimate.
  • I for Independent” suggests that we create stories that can be independently estimated and implemented. While this is mostly true, it is a simplification of reality: sometimes the cost of a story depends on the order of implementation or on what else is implemented. It may be hard to capture that in estimates.
  • Factors that make it hard to estimate are not stable over time. So even if you’re able to take all those factors into account, you also have to account for their instability.

Is estimating hopeless? If you think estimation is a simple process that will yield an exact (and correct!) number, then you are on a futile quest. If you just need enough information from estimates to guide decisions, you can usually get that.

Some projects need detailed estimates, and are willing to spend what it takes to get them. In general, though, Tom DeMarco has it right: “Strict control is something that matters a lot on relatively useless projects and much less on useful projects.”

Where does that leave things? The best way is to use as light an estimation process as you can tolerate.

We’ll explore three approaches: counting stories, historical estimates, and rough order of magnitude estimates.

Simple Estimates: Count the Stories

More than ten years ago, Don Wells proposed a very simple approach: “Just count the stories.”

Here’s a thought experiment:

  • Take a bunch of numbers representing the true sizes of stories
  • Take a random sample
  • The average of the sample is an approximation of the average of the original set, so use that average as the estimate of the size of every story (“Call it a 1”)
  • The estimate for the total is the number of stories times the sample average

What could make this not work?

  • If stories are highly inter-dependent, and the order they’re done in makes a dramatic difference to their size, the first step is void since there’s no such thing as the “true” size.
  • If you cherry-pick easy or hard stories rather than a random set, you will bias the estimate.
  • If your ability to make progress shifts over time, the estimates will diverge. (Agile teams try to reduce that risk with refactoring, testing, and simple design.)

I’ve seen several teams use a simple approach: they figure out a line between “small enough to understand and implement” and “too big”, then require that stories accepted for implementation be in the former range.

Historical Estimates (ala Kanban)

For many teams, the size of stories is not the driving factor in how long a story takes to deliver. Rather, work-in-progress (WIP) is the challenge: a new story has to wait in line behind a lot of existing work.

A good measure is total lead time (also known as cycle time or various other names): how long from order to delivery. Kanban approaches often use this measure, but other methods can too.

If we track history, we can measure the cycle times and look for patterns. If we see that the average story takes 10 days to deliver and that 95% of the stories take 22 or fewer days to deliver, we get a fairly good picture of the time to deliver the next story.

This moves the estimation question from “How big is this?” to “How soon can I get it?”

When WIP is high, it is the dominant factor in delivery performance; as WIP approaches 0, the size of the individual item becomes significant.

Rough Order of Magnitude

A rough order of magnitude estimate just tries to guess the time unit: hours, days, weeks, months, years.

You might use such estimates like this:

  • Explore risk, value, and options
  • Make rough order of magnitude estimates
  • Focus first on what it takes to create a minimal but useful version of the most important stories
  • From there, decide how and how far to carry forward by negotiating to balance competing interests
  • Be open to learning along the way


Stories are estimable when we can make a good-enough prediction of time, cost, or other attributes we care about.

We looked at approaches to estimation and key factors that influence estimates.

Estimation does not have to be a heavy-weight and painful process. Try the lighter ways to work with estimates: counting stories, historical estimates, and/or rough order of magnitude estimates.

Whatever approach you take, spend as little as you can to get good-enough estimates.

Related Material

Postscript: My thinking on this has definitely evolved over the years, but I’ve always felt that Small and Testable stories are the most Estimable:)

Review: Accelerando, by Charles Stross

Accelerando Accelerando, by Charles Stross. Ace, 2006.

This novel explores the Singularity: What happens when AI, uploading, and other variations of humanity (or non-humanity) start to happen. I was worried that a novel on this subject would basically be, "Computers take over, and who knows what they're thinking?" Instead, we follow the protagonist's family through generations, and get a plausible and enjoyable flavor of what the Singularity could mean. 

Review – Ruled Britannia (Turtledove)

Ruled Britannia

Ruled Britannia, by Harry Turtledove.

This is another of Harry Turtledove's alternative history novels. In this case, the Spanish Armada succeeded in the late 16th century, and Spain has taken over England. This is the time of Shakespeare and Queen Elizabeth, and Shakespeare ends up involved in politics and intrigue – what will become of England?

It's good fun – lots of small changes in plays and titles, and lots of witty dialog and snaps in a Shakespearean style.

Review – Musashi

The Clockwork Man  

Musashi, by Eiji Yoshikawa. Kodansha International, 1995. 

Swordplay, true love, and a man climbing up the side of a cliff. It's not the Princess Bride; it's the story of Musashi, the most famous swordsman of Japan.

This is a romanticized, historical fiction novel. Musashi started out on the losing side of a battle, and spent his life seeking the true way of the samurai. The story has many coincidences and near misses (though fewer than the average episode of 24), but the charm and fun overcomes that.

Review – C.S. Lewis Space Trilogy

Out of the Silent Planet, by C.S. Lewis
A linguist (Ransom) is kidnapped and ends up on another planet. (The science is wobbly, but it's a classic travel story stretched to unexplored worlds). How will he work with humans or with those of that world?

PerelandraPerelandra , by C.S. Lewis
Ransom has to go to Venus, a world where paradise hasn't fallen. He's in a battle with evil to prevent the fall from grace. Of the three books, the religious overtones are strongest in this one.

That Hideous StrengthThat Hideous Strength, by C.S. Lewis
Lewis explores evil that sneaks up on you. A newly married man gets involved with a social and political group that is actually ruining England. How do you extract yourself when you realize you're deep inside something wrong? (I'm reminded of both 1984 and Mrs. Frisby and the Rats of NIMH, but they were written after this one.) As often happens with Lewis, there's a combination of Christianity and mythology that comes through.

I enjoyed these, but it definitely requires accepting that they are written from a particular moral stance. The story moves forward well and makes a fairly quick read.

Review: The Clockwork Man (Jablonsky)

The Clockwork Man  

The Clockwork Man, by William Jablonsky. Medallion Press, 2010.

Late in the 19th century, the world's best clockmaker creates a clockwork man (who doesn't like being referred to as a robot). He (it?) lets himself run down, and wakes up more than a century later, to figure out what happened before and where he should go next.

It's a short read, written in the form of a diary or journal. I thought the ending was foreshadowed too much, but it was an enjoyable story and I'll look for this author again.

(I saw this tagged as "steampunk" on Amazon; I hadn't run across that category before but it fits.)

Review – Mabinogion Tetralogy, by Evangeline Walton

Mabinogion Tetralogy, by Evangline Walton. Overlook. 2003.

This is an under-appreciated work of fantasy. It’s based on the Mabinogion, a set of Celtic myths found in Welsh documents. The version I bought 25 years ago was in four separate volumes, but I’ve linked to a recent version with everything in one volume.

The Prince of Annwn tells of a prince who helps a mysterious Gray Man. A few years later, the druids blame him for troubles in the land, and he vows to take a wife. (Throughout these stories, you see interplay between old tribes and new tribes, men and women.)

The Children of Llyr starts with a time when people are starting to figure out how men are involved in procreation. Inheritance had passed down to one’s sister’s children, since you knew they were related. This is a story of brothers and the troubles between them that resulted in death and change.

The Song of Rhiannon ties the first two stories together, following the story of one of Llyr’s children.

Finally, The Island of the Mighty tells of a sorceress who tried to control someone’s destiny but found she could not.

If you enjoy fantasy, or would like to sample it without picking up something as intense as Tolkien, these are an excellent read.

Review – Mythology, by Edith Hamilton

Mythology, by Edith Hamilton. Back Bay Books, 1998. 

Hamilton works through the basic myths, mostly from Greek and Roman sources. Topics include “The Gods, the Creation, and the Earliest Heroes”, “Stories of Love and Adventure”, “The Great Heroes before the Trojan War”, “The Heroes of the Trojan War”, “The Great Families of Mythology” (Atreus, Thebes, and Athens), and “The Less Important Myths”. She closes with about 15 pages of Norse myths (just a taste). The stories are typically brief, easy reads in a fairly modern style. I was struck by how she describes the difference between different authors (though I’m not to the point of distinguishing them myself). 

Review- The Dark Tower and Other Stories

If you want to complete your collection of C.S. Lewis fiction, this volume will do that. (Note that about 1/3 of it overlaps with the stories included in Of Other Worlds by the same publisher and editor). “The Dark Tower” is a partial story (about 70 pages) about a parallel universe; the characters overlap a little with the “Space Trilogy.”

“The Man Born Blind” is a story of a man who is given sight and wants to understand light. “The Shoddy Lands” is a short study in perspective. “Ministering Angels” is about a trip to Mars (and reminds me of some of Isaac Asimov’s humor pieces). “Forms of Things Unknown” is about a dangerous trip to the moon. “After 10 Years” is a partial story (about 20 pages), a followup to a Greek myth. 
In general, the science fiction is “just” a carrier to get us to places unknown rather than being integral to the story. The stories themselves are good (though sometimes rough), laced with myth in his trademark way.

Review – Legends of Dune (series)

The Legends of Dune series: The Butlerian JihadThe Machine Crusade, and The Battle of Corrin (or the Box Set), by Brian Herbert and Kevin Anderson. Tor Books, 2003-2005. 

This series is called “Legends of Dune.” It’s set thousands of years before “Dune,” when people were working out all the technologies and schools that showed up there. Find the roots of the Harkonnens, Atreides, Corrinos, and more. Being set so much earlier does free the authors up some; any discrepancies can be put down to the confusions of time. 

The first book has humans against robots and cymeks, and shows why the Butlerian Jihad got its name. The second book follows the struggles of the cymeks, early space-folding, and a human who grew up with the robots. The third book traces the ultimate battle with robots.

These books use the Dune universe, but I found the style closer to Asimov than Herbert (with lots of bouncing around following various characters and threads). It was enjoyable enough, but don’t expect the equal of Dune.

Review – The Golden Compass, The Subtle Knife, and the Amber Spyglass

The Golden Compass, The Subtle Knife, and The Amber Spyglass. (3-volume set) By Phillip Pullman. Laurel Leaf (pub.), 2003.

It’s hard to review this series without giving away too much, but here goes:

The Golden Compass:This story is set in a world where part of your personality takes the form of an animal (called a daemon, but not like the ones in Unix:). We meet a girl named Lyra in Oxford in another world. She acquires an amazing compass that helps guide her on a dangerous journey through what we’d call a fantasy realm because it has witches and armored bears.

The Subtle Knife:
Lyra ends up in a different world, and meets Will from our world. They quest on, meeting with enemies and unexpected allies in different worlds. Will takes on new missions.

The Amber Spyglass:
The story continues and concludes with Lyra, Will, and many others. We’re treated to an unlikely evolution that sounds almost plausible, another amazing tool, and a struggle affecting angels and demons, the living and the dead.

Overall, the series is compelling and interesting. It blends an interesting mix of scientific and religious ideas, with a notable anti-organized-religion bias that will put some people off.

Review – Dune

Dune, by Frank Herbert
When I think through my favorite science fiction, I always come back to Dune. It’s a sweeping and powerful mix of intrigue with ecology. Herbert creates a unique but plausible world, and explores the forces that move through it. This book is the starting point of a series, but can be read on its own. If you haven’t read Dune, you’re in for a great read; if you have, maybe it’s time for another go.

Review – Frankenstin (Koontz)

Frankenstein (Dean Koontz et al.):
Prodigal Son (vol. 1)
City of Night (vol. 2)
Dead and Alive (vol. 3)

(I can’t resist posting this review on Halloween.)

A pair of detectives are working to solve murders and other other mysteries. Frankenstein or his monster are involved. The dialog is snappy, the characters are fun, and it’s a page-turner (as you’d expect from Dean Koontz). The third (and last) book was weak, with a lot of chasing around to close loose ends, but up till there it was a very enjoyable series. (The third book changed it from a keeper to an airport read for me. )

Review – The Well at the World’s End

The Well at the World’s End (volume 1 and volume 2), by William Morris, Borgo Press, 2000. [Originally published 1896!]
This is considered the first fantasy novel set in its own world. William Morris was an artisan (leader of the Arts and Crafts movement), decorator, poet, producer of the first recliner chair, and all-around multi-talented guy. The story is of the heroic quest for the Well that gives long life. With its knights, archaic language, and reliance on a little more luck than seems common, you can see its influence on C. S. Lewis, Tolkien, and the rest of the fantasy genre. (Reviewed Oct., ’09) [The version I had was in one volume; I think these two are the equivalent together.]