Measuring and estimating

Units go together in powers of ten:

Length	Weight	Time
1 km = 1 000 m	1 kg = 1 000 g	1 h = 60 min
1 m = 100 cm	1 g = 1 000 mg	1 min = 60 s
1 cm = 10 mm

To estimate is to guess a sensible value without measuring: a door is about 2 m tall, an apple weighs about 150 g, brushing your teeth takes about 2 minutes.

Guess first, then measure. Write your guess down before you measure — otherwise you fool yourself.

Thing	My guess	Measured	Error
the length of the table	120 cm	138 cm	18 cm
my water bottle	400 g	520 g	120 g

After five goes you get noticeably better. That is exactly how a model is trained: guess, compare with the answer, adjust.

The order of magnitude is often enough: is it 10, 100 or 1 000? If your calculation says a cat weighs 400 kg you have gone wrong somewhere.