I was teaching a knitting class the other day when much to the surprise of my students, a math class broke out.
A student who will shall call G. is working on a sweater. Because G. is a sensible sort (who has known me long enough to know I would tell her off if she didn't...) she swatched.
The gauge required is 8 sts/4 inches. Before she measured, G. did something we all would do: she divided. 8 sts for 4 inches - that's 2 sts an inch.
So G. measured her swatch and saw 2 stitches per inch. She was good to go! She had needles, yarn, and a plan for a fabulous quick knit jacket for early spring!
I measured 4 inches, carefully counted the stitches, and got 9 stitches in 4 inches. 9 stitches in 4 inches is definitely not 8 stitches. 9 stitches in 4 inches is 2.25 stitches an inch; 8 stitches in 4 inches is 2 stitches an inch. Now, the difference is only a measly quarter of a stitch. G. had measured 1 inch, and in the fuzzy, fluffy yarn G is using, that quarter of a stitch over an inch was pretty hard to see.... it's a mistake we might all make, I don't blame her one bit.
G. is a lovely person, and she would never say so, but I had the distinct sense that she felt I was making a mountain out of a molehill: she really couldn't see why I was so worried about that quarter of a stitch.
But here's the problem: what seems like a rounding error - a tiny little fraction of a stitch - can make a HUGE difference. Enormous. Ludicrous, even.
The finished size of the garment is 44 inches, and you cast on 88 stitches for the full body circumference. 88 sts at 2 sts per inch = 88/2=44. Perfect.
But at 9 stitches per 4 inches... 88 sts at 2.25 sts per inch =88/2.25 = 39.1 inches around.
Yes, that's right. A quarter of a stitch difference makes for a nearly 5 inch difference in the finish garment.
It turns out that your parents were right - math does matter!