Standard Time from a Time Study

A stopwatch reading is not a standard. Two things have to happen to it first. Rate the operator against normal pace, because the person you timed may have been working at 110% or at 85%, and multiply. Then add an allowance for rest, personal needs and unavoidable delay, because nobody works 480 minutes out of an 8-hour shift. Two minutes observed, rated at 110%, with a 15% allowance, gives $2.00 \ imes 1.10 \ imes 1.15 = 2.53$ minutes allowed.

Performance rating is the soft joint in an otherwise hard method, and it is why time study is contentious. It is a trained human judgement, calibrated against films of "normal" pace such as dealing a deck of cards in 0.50 minutes, and two rated observers can differ by five points on the same operator. That variation flows straight through to the standard, and from there into piece rates and costings.

Watch the allowance convention. Most North American practice applies the allowance to the normal time as written here, so 15% adds 15% of 2.20 minutes. Some texts, and some union agreements, express the allowance as a fraction of the total shift instead, in which case the multiplier is $1/(1-A)$ and 15% becomes a 17.6% addition. The two differ by enough to matter on an incentive rate, and the contract usually specifies which one applies.