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How Science Works

Accuracy, Precision& Sensitivity

Accuracy

You will normally be taking three or more readings of the same measurement, and using the mean of these values as your result.

Accuracy is how close the measurementis to the true value of the variable.

How to Achieve Accuracy

Use a well calibrated, precise and sensitive instrument.

Make sure that the measuring instrument has small enough divisions along the axis.

You may need to set an instrument to zero before use.

How to Achieve Accuracy

Take the readings as carefully as possible.

Repeat the measurements at least three times.

Look out for anomalous results – odd ones which don’t fit the trend / pattern

Repeat a measurement if necessary.

Precision

A precise measurement is one in which the repeated readings are grouped closely together - there is very little spread around the mean value..

Precision

PrecisionA precise measurement is one in which the repeated readings are grouped closely together - there is very little spread around the mean value.

The Difference Between Accuracy & Precision

Measurement can be precise but not necessarily accurate...

The Difference Between Accuracy & Precision

If three measurements of the same length are taken very carefully from the wrong end of a ruler, the measurements will be precise but will be inaccurate.

Measurement can be accurate but not necessarily precise... If three measurements of the same length are taken carelessly

they will not be precise.The mean of these measurements may be close to the true length - therefore accurate.

You should always try to achieve accuracy and precision -

In this animation, the Bull’s eye represents the true value of the variable, and the darts represent the measured values.Throw the darts, then drag the labels to the correct place...

The Difference Between Accuracy & Precision

Precision of InstrumentsA precise instrument will give very close readings for the same value of a variable, e.g. if a 1kg mass is repeatedly weighed on a precise balance, it will give very close (if not exactly the same) readings each time.

Precision in instruments is about the quality and reliability of the instrument.

Using a more precise instrument will lead to more precise measurements, because precision is about reducing the range of results when measuring the same value of a variable.

Sensitivity of Instruments

A set of scales calibrated in milligrams is more sensitive than a set of scales calibrated in grams.

Sensitivity is the smallest change that an instrument can measure.

Using a More Sensitive InstrumentA more sensitive instrument will lead to more precise measuring, because there are more divisions to assist in reading the scale…

Measurements for the above reading could be estimated as 0.7, 0.8 or 0.9

A more sensitive instrument has extra divisions along the scale. Measurements will now be either 0.8 or 0.9

Sensitivity of InstrumentsA more sensitive instrument has smaller divisions to measure smaller changes in the variable...

More SensitiveLess Sensitive

Look at these four scale readings, then drag the scales into the order of their sensitivity...

Sensitivity of Instruments

Reliable MeasuringThe smallest unit to which you can reliably measure, will be the smallest division on the apparatus you select.

Example:

If you want to measure to the nearest tenth of a degree, your thermometer must be calibrated in tenths of degrees, like this one.

Litmus Paper

Drag the paper into the beaker to see the colour change...

If in your experiment you only need to identify a substance as acid

or alkali, litmus paper is sensitive enough.

Litmus Paper

Drag the paper into the beaker to see the colour change...

If in your experiment you only need to identify a substance as acid

or alkali, litmus paper is sensitive enough.

Universal IndicatorUniversal indicator (UI) is a much more sensitive indicator.It not only indicates if the substance is acid or alkali, but also how strongly acid or alkali the substance is.Drag the paper into the beaker to see the colour change...

Universal Indicator

Even greater accuracy can be achieved with electronic pH meters.

Universal indicator (UI) is a much more sensitive indicator.It not only indicates if the substance is acid or alkali, but also how strongly acid or alkali the substance is.Drag the paper into the beaker to see the colour change...

End of Show

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