ACCURATE, PRECISE, RELIABLE

AccuracyPrecisionReliabilityTo learn the meaning of:

Taking measurements: MEANThe results of an experiment were:10.1 s, 10.2 s, 9.9 s, 10.0 s, 10.3 sMean is: 10.1+10.2+9.9+10.0+10.3 = 50.0 = 10.1 s5 5Why is it a good idea to take the mean?Because it improves the reliability of the results.Taking the mean does not improve accuracy.

How can an experimental result be made reliable?Repeating the experiment.Taking a large sample.Removing any anomalous results (leave some out because they did not fit a pattern).Eliminate other variables.Question

Definitions:Accuracy and Precision... sound the same thing.......is there a differnce??

In your experiments you need to consider the accuracy of your measuring instrument.Definitions: AccuracyFor exampleAn expensive thermometer is more likely to be more accurate than a cheap one.It will give a value nearer to the true value.It is also more likey to be more sensitive (with a better resolution). It will respond to smaller changes in temperature.

As well as accuracy, precision is also importantPrecision is connected to the smallest scale on the measuring instrument.Definitions: Precision

will give greater precision than one with cm scale Definitions: PrecisionFor example: using a ruler with a mm scale

also gives a consistent reading when it is used repeatedly for the same measurments.Let us follow an example: A Precise instrument:

A beaker is weighed on A three times.The readings are: 73 g, 77g, 71 gSo the RANGE is : 71 – 77 = 6gIt is then weighed on B three timesThe readings are: 75g, 74g, 73 gSo the RANGE is : 75 – 73 = 2gPrecisionABBalance B has better precision.Its readings are grouped closer together.

Can be quantified by calculating:RangeCan be improved by:Better instrumentsPrecision

Suppose you are measuring the length of a wooden bar:Accuracy compared with PrecisionThe length has a true value.And we can take measurements like this: Let’s look at 3 cases. 0True value

Precise(grouped)But not accurateAccuracy compared with PrecisionTrue valueAccurate(the mean)But not preciseAccurate and precise

Accuracy means how close it is to target.Remember:Precision means how close repeated measurements are.

Consider:

1. Heather and Jason determined the density of a liquid three different times. The values they determined were2.84 g/cm32.85 g/cm32.80 g/cm3 The accepted value is known to be 2.40 g/cm3.Are the values that Heather and Jason determined precise? Explain. Yes. Repeated measurements are close to each other. b) Are the values accurate? Explain. No. Values are not close to the accepted value.Question:

The leaves of a particular species of plant vary in size. This variation is caused by both environmental and genetic factors. A student investigated how light intensity affects leaf size at two sites. 1. The student measured a large sample of leaves at each site. Give two reasons why the student measured a large sample. (2)i. Make sure the sample was representativeii. Produce a reliable mean; iii. Reduce the effect of extreme values / reduce the effect of / anomalies / identify anomalies;Question

Do not accept ‘make sure sample was reliable/ precise / accurate’.2. The student collected the leaves at random. Explain why. To avoid bias;Question

3. The student calculated the standard deviation of the sample of leaves. Why is it useful to calculate the standard deviation? Give measure of spread of variation in data;Question

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