02 June 2009

145. Precision vs. Accuracy

ALERT: SEWING GEEK-OUT FOLLOWS....... Proceed with caution and patience...

Sometimes you understand a concept and sometimes you really get it. Since I learned about precision and accuracy in elementary school science, until the past few weeks, I have merely understood the concepts of and difference between precision and accuracy. Now that I've had some time working with flat patternmaking and draping I feel that I really get it.

Wikipedia has a great entry with visuals on precision and accuracy, so there's no use paraphrasing. Here's just one real-life example:

I traced around my sloper, rotated darts, added ease, etc., then traced around that pattern onto paper I would pin down to cut the fabric. Well, if I added even 1/16" around the sloper the first time I traced, then added another 1/16" around the modified pattern to the usable pattern, I added 1/8" total size all around over the sloper. Doesn't sound like much, but if you do this (*ahem*, does it sound like I have personal experience with this?) on a fitted princess seam bodice with a back zip, that totals 14 seam allowances for a total increase of 7/8" around the width of the bodice. My work was precise on each pattern piece, but the end result was very inaccurate when you compared the sewn-up sloper to the sewn-up newest pattern.

Conversely: one insomniac night, I draped a top on Clementine for the most accurate fit and compared the pieces to the flat pattern. Not even close in some areas. How bizarre and non-intuitive, right? Very accurate fit, but completely imprecise compared to the flat pattern.

For those of you who have seen The Fashion Show -- I think in episode 3, when Reco was team leader, he drew a pattern piece freehand, just eyeballing it on the paper. I know the resulting garment was spot-on, and I'd bet his results were both accurate and precise. I bet that mastery takes years and years and maybe even a special gift.

Precision vs. accuracy. Flat pattern vs. draping. I see this is what my sewing guru meant when she said that many patterns are best designed using both. (BTW, if you've read this far, I'm by no means implying that draping can't be precise. I guess I'm just not a precise draper yet.)


  1. I welcome the dialogue about this... You know, I'm sure, that part of the reason I do "one of a kind" is to escape the scrutiny of repetition and precision. The little flaws are part of the beauty, right? But I secretly harbor a desire for total accuracy... which I hope will one day materialize.

  2. Yeah, I just read this to pretend that I'm part of the real sewing world, but I was pretty much lost after "elementary school science"! HAHA!

    I have a vague idea of what a sloper is. That it's some sort of thing that represents your actual body. Is it flat?

  3. Such a good explanation of the terms! I definitely don't worry too much about precision because I just make sure it's good enough for me, but if I were sewing for others and having to fit their bodies it would be a different story.

  4. I believe over time, you understand your own body and feel comfortable with the pattern changes. I like to do flat pattern measurements and if it is a difficult design or I am not quite sure...I will whip up a muslin and pin/drape if there is a problem. The hard part for me is to take all the "tucks" from the draping and transfer it to the pattern. Does that make sense? I definitely don't feel comfortable sewing for other bodies right this minute!!

  5. That little demonstration by Reco is why I think he's so gifted - it does take a lot practice to do that and a gift too!

  6. So...what I'm getting from this is...unless my measurements are close to those of the fit model used in the design of the trouser patterns I intend to one day sew up - I have very little chance of making a pair of well-fitting trousers without at least a couple of (dozen) muslins...

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  8. Is this where using a mechanical pencil with super skinny lead comes in handy?

    What should I call my self-drafted patterns? Very accurate fit. But precise or otherwise? I have nothing with which to compare the pattern pieces.

    Let's say, I draft the pattern again but this time around using a mechanical pencil with super skinny lead, which really makes no sense since I'm not tracing, but at least, all the cutting lines are equal width, and that I cut my pattern pieces accurately. And a third time around using a pattern drafting software. (I have not even an itty bitty bit of authority on any pattern drafting software, but I worked with CAD before children came along. Let's pretend I know how to use one fairly sophisticated.) And a fourth, draping. (Again, let's pretend.) Add to all this how I've kept the fabric from distorting through the sewing process and how accurate my needle is to the stitching line.

    Then perhaps now, I can say, how precise?

  9. unfortunately I am neither accurate nor precise.

  10. As someone who wants to get serious about my clothing sewing, this is fascinating stuff if only because I know I need to strive for that precision (whether altering patterns, selecting sizes, what have you) in a way that hasn't come naturally--to some extent I'm sure because the training wheels haven't mentally come off yet. But they're going to, by golly!

  11. Regarding drafting vs draping, that's what my teacher said too. If he drapes, he usually tweaks the patterns afterwards using flat pattern making techniques. Although he is very "whatever rocks your boat" in his approach, i.e. whatever gives you the best result

    Regarding accuracy I have the same problem, I usually add a little here and there, and it does add up. So I try to check the pattern against my original sloper draft to make sure I'm not too off.


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