Globetrotter is making the excellent point that differences in coins should be quantified. "Small head" is about as informative as "stray dog". However, 2 mm from crown to edge is instantly verifiable in countries using the decimal system. A few thoughts.

Measurements are as good as the measuring instrument. Plastic callipers are horribly imprecise, because plastic bends. Laser distance measurers are horribly precise. The difference used to be price. No longer. High precision electronic scales are commonplace and cheap. Electronic callipers have come down in price to the point where they they have become a solution for the mathematically or eyesight challenged.

Good measurements must be reflected in good notation. Everyone understand the use of the zeroes in 0.09 cm, but what is the use of the zeroes in 9.00 mm? The zeroes indicate the precision of the measurement. A notation of 9 mm means that the diameter of the coin is between 8.9 and 9.1 mm, while 9.00 mm means that the diameter is between 8.996 and 9.005 mm.

Now consider the precision question again. Weigh the same coin 5 or 10 times. If your scales give you the same value at least 4 or 8 times, use all the digits on the read-out. Otherwise, use the last digit for rounding only. Do the same test for your callipers. Chances are that your callipers are less precise. Why? Because YOU position the coin in the calipers and gravity positions the coin on the scales.

Official weights are not one number, but a formula that comes down to a weight plus a margin above and below (the remedy). Coins that are outside the remedy should be rejected. A remedy is stil necessary. Take a number of coins struck according to the same specifications (e.g. a series of commemoratives) and you will find slightly different diameters and weight.

Observed coin weights are imprecise, because of wear and dirt. Observed weights (plural) are therefore a second choice for official weight. One observed weight only is third choice. The best way to divine the official weight is to have 1000 copies, weigh them, determine average and standard deviation (Excel will do this for you.) But what if you do not have 1000 coins? Weigh as many as you can, but at least five coins of the same type and take the average. Now find the weighing system used in the country of the coin at the time it was struck and express the average in that system. Round (preferably up) to the nearest likely weight.

When it comes to differences in design, I agree that quantifying them is important, especially for those who have only one copy and wonder which variety it is. Nevertheless, as collectors, we should not lose sight of other aspects, like spacing and alignment. I don't think designs are subtly changed because someone said "Mmmm, I think that lion should have a 1.7 mm chest". More probably, someone said "Mmmm, that lion looks too skinny. Look at how thin it is compared to the pedestal".

Peter