Comments on: Using metric - accuracy v precision http://www.metricviews.org.uk/2007/07/02/using-metric-accuracy-v-precision/ Commentary on the measurement muddle in the UK Tue, 06 Jan 2009 15:42:25 +0000 http://wordpress.org/?v=2.0.5 by: Dave Brown http://www.metricviews.org.uk/2007/07/02/using-metric-accuracy-v-precision/#comment-2371 Thu, 05 Jul 2007 11:51:17 +0000 http://www.metricviews.org.uk/2007/07/02/using-metric-accuracy-v-precision/#comment-2371 This seemingly technical issue is quite important in persuading people to use the metric system. A lot of resistance comes from people who think that imperial measurements need to be exact, so translate litterally to metric and then wonder why they get numbers they can't remember. For example, if a recipe calls for 8oz of flour, and you litterally translate that to 227 g and then try to weigh that quantity, you're going to find it really tricky. In fact you could easily get away with anywhere between 200 and 250 g, because cooking doesn't require that level of precision. It's the same in other areas - you don't need 25.4 mm, normally 20, 25 or 30 mm is precise enough to replace an inch; 250 or 300 ml is often precise enough to replace half a pint etc. This seemingly technical issue is quite important in persuading people to use the metric system. A lot of resistance comes from people who think that imperial measurements need to be exact, so translate litterally to metric and then wonder why they get numbers they can’t remember. For example, if a recipe calls for 8oz of flour, and you litterally translate that to 227 g and then try to weigh that quantity, you’re going to find it really tricky. In fact you could easily get away with anywhere between 200 and 250 g, because cooking doesn’t require that level of precision. It’s the same in other areas - you don’t need 25.4 mm, normally 20, 25 or 30 mm is precise enough to replace an inch; 250 or 300 ml is often precise enough to replace half a pint etc.

]]> by: Mike http://www.metricviews.org.uk/2007/07/02/using-metric-accuracy-v-precision/#comment-2363 Wed, 04 Jul 2007 21:23:15 +0000 http://www.metricviews.org.uk/2007/07/02/using-metric-accuracy-v-precision/#comment-2363 Also as a more graphic demonstration of the difference between accuracy and precision (one that we were taught when I first started studying Chemistry at uni) one can imagine playing darts and aiming for the bullseye. A precise and accurate turn would result in the three darts being clustered closely around the bull. A precise but inaccurate turn would result in all three darts being clustered away from the bull. An accurate but imprecise turn would for example have the three darts spread around the edge of the board so that their average position was the bull. I always found this an easy way of remembering the difference. In summary, in scientific terms, precission refers to the 'reproduceability' of the result but makes no reference to how correct it is. Accuracy refers to the 'correctness' of the result, even though it may have been achieved as an average of imprecise measurements. Also as a more graphic demonstration of the difference between accuracy and precision (one that we were taught when I first started studying Chemistry at uni) one can imagine playing darts and aiming for the bullseye.

A precise and accurate turn would result in the three darts being clustered closely around the bull.
A precise but inaccurate turn would result in all three darts being clustered away from the bull.
An accurate but imprecise turn would for example have the three darts spread around the edge of the board so that their average position was the bull.

I always found this an easy way of remembering the difference. In summary, in scientific terms, precission refers to the ‘reproduceability’ of the result but makes no reference to how correct it is. Accuracy refers to the ‘correctness’ of the result, even though it may have been achieved as an average of imprecise measurements.

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