Sometimes swatches lie

I decided to knit myself a Plaited Tam, but I wanted to try a different yarn; even though I loved the SweetGeorgia Yarns Superwash Sport I used for the sample, it's not available locally, and can take up to four weeks to arrive if ordered from the SweetGeorgia website (since it's dyed to order).  So I decided to make things a little challenging for myself, and use a yarn of a different gauge:  Cascade 220 Superwash, which is more of a light worsted weight.  I did a swatch of a couple of repeats of the Plaited Tam stitch pattern, including the ribbing, and since one repeat measured 2 to 2 ¹/₄ inches at the bottom edge, after blocking, I figured the larger size, with 8 repeats, would give me a finished circumference of 16-18 inches.  Sounds good, right?  (I didn't care so much about the finished diameter of the tam-- a little oversized was fine with me).

I used Color #1910, a heathered aqua (color in photos is off-- it's much prettier in real life), and after blocking over a large plate, it was a bit oversized, at around 12" diameter.  But the ribbing part was only around 16" when measured while the tam was lying flat, so...

it was only when I tried to put it on that I realized that it was definitely too large!  I could not get it to stay on my head without it slipping down over my eyes, unless I perched it back behind my hairline (and then it looked like a wooly shower cap).

Lesson learned: even if a swatch seems to measure properly when lying flat, it is essential to consider how the fabric will behave when it's worn; in this case, the ribbing stretched too much under its own weight to hold the hat on my head.  I should have measured the swatch slightly stretched; when I did, one repeat measured 2 ³/₄ inches, which translates to a circumference of 22" (and my head is 21").  

Next:  I try knitting the smaller size!