Skill Guide : O
Obtaining Evidence

1. Overview

1.1 Typical Results Outline

Control Variables	A list of control variables and their values, including units.
Table Title	A short descriptive title.
Tables	Neatly drawn. Logical layout.
Table Header	Contains variable name and unit.
Table Body	Contains data, to correct precision. Contains repeat data and an average. Does NOT contain units.
Observations	Relevant observations about the procedure and the results.
Accuracy	An estimation of the accuracy of measurements.

1.2 Writing up your results

Results must be presented neatly and clearly. Anyone who looks at your results should immediately be able to see what you have done and what the main patterns in the results are.

It is almost impossible to get a high mark for your evidence if you did not get a high mark for the planning stage.

The above outline shows how the results for a typical experiment would be organised. The format for something like a Biology fieldwork investigation would contain much more in the way of descriptive observations and drawings. These too would need to be presented in a logical order with selection of relevant information only. If there are a lot of results then it may be a good idea to write a contents page cross-referenced to all the diagrams and tables.

2. Obtaining Evidence

2.1 Introduction

After all the hard work of the Planning Stage you should be ready to collect your data. If you have prepared well and carried out trial experiments there shouldn't be too many unexpected difficulties. Your evidence is judged on its quality, not its quantity. Evidence must be accurate and reliable and must cover a sufficient range to test your prediction.

2.2 Setting-up

Make life easier for yourself by taking a bit of care in setting up your apparatus. Make sure it is neatly and safely laid out and that it is easy to take and record measurements. If the experiment requires more than one person make sure that you work effectively together. Avoid working with anyone who does not have the same objectives as you; i.e. to obtain quality results and get a high mark. Remember to take time to get things right, but make sure you don't waste time.

Check that each bit of equipment is in good working order before you use it. Here are some common faults to check for;

safety precautions

implemented as planned;

glassware

clean, no chips or cracks;

solutions

correct concentration;

stopwatch

start/stop button works smoothly;

electrical connecting wires

check with a resistance meter for breaks in the wires;

electrical connections

must be clean, and not loose;

power supplies, batteries

check voltage with a voltmeter;

electronic instruments

check battery is not low, fuse is not blown;

clamps and stands

must be secure and screwed up tightly;

unstable equipment

use G-clamps to secure it to the bench;

2.3 Carrying out

Follow the procedure you have planned but never be afraid to modify your procedure as you go along if this means you get better results. You must always be on the look out for faulty or incorrect readings. This may be due to simple human error or problems with the equipment. Whatever the cause you must repeat the measurements immediately. To get top marks you are not expected to have any faulty measurements. You are expected to rectify the faults and only include the good, true data. Nobody is interested in knowing what the reading was on the occasion your "hand slipped", or whatever.

There is usually only one way to tell if your results are "good enough" and that is to analyse them. You may be able to do this as your experiment progresses. You may need to calculate further values but you definitely will need to plot a rough graph. A rough graph means you quickly put the scales on a bit of graph paper and plot the points (accurately!) to see what the pattern is like. Large gaps in certain sections of the graph? Take more in-between values. A few points seem to be a long way from the line of best fit? Go back and double-check these values and the ones around them. Range too narrow? Take some additional results to widen it. Found an anomaly? Great! Go and investigate it.

It is no good waiting until all the equipment is packed away to decide that your results are not quite good enough; it is then too late.

Descriptive observations may be very important as well as direct measurements. Make a note of any particular technique you use to improve the accuracy of your results. Write down any difficulties you have in obtaining individual results or groups of results. Make a note (mental, at least) of anything odd which happens. You may be able use it later to help explain that kink in your results.

2.4 Recording

Record measurements exactly as they are measured before any sorts of conversion are carried out. It is all too easy to make errors by carrying out conversions in your head. Write the figures down, then convert them if you need to. This way if you make a mistake you can check back to the raw values and make the correction. Don't forget to record the units for the measurements, and to make a note whenever you switch scales and change to a bigger or smaller unit. Results can be written down neatly in rows and columns without spending time drawing up a table. Save your drawing skills for your final write-up.

2.5 Observations

Scientific observations must be relevant, clearly expressed and contain an appropriate amount of detail. The observations required will depend on the investigation. The following list gives you some ideas to think about.

drawings

...neat and in pencil, labelled, large and clear;

scale

Biological drawings must have a scale shown. This is particularly important when using a microscope;

shape

..and is it 2 or 3 dimensional;

colour

Describe a particular shade of colour. Use a standard colour chart with names for each shade. Is the colour uniform or does it vary?

texture

..and feel;

smell, sound

similar to...;

changes

Changes over time, how long it takes to change;

association

Occurrence with other things, or at the same time as other events;

physical/chemical conditions

Changes in size, state (solid, liquid, gas), pressure, energy, temperature;

pH

Changes in acidity or alkalinity;

2.6 Repeating

All your measurements and observations must be repeated. The number of repeats will depend on the type of experiment and the time available. The minimum number of repeats (obviously) is two good sets of results. "Bad" sets of results do not count as a valid repeat. Experiments that are quick to carry out should be repeated three times. If there is a large variation between individual results then more repeats are needed to produce a more reliable average.

3. Presenting Evidence

3.1 Presentation of Results

You may have the best results in the world but if you don't present them correctly you will slip down the mark scale. Here is a check list of what you must do.

• State the value for each control variable.
• Draw the main results table neatly with a ruler.
• Give a short title for the table.
• Each column heading must have a name and unit.
• Include repeated values.
• Values of the independent variable must be in ascending or descending order.
• All values must be given to the correct precision, including trailing zeros.

3.2 General format

3.3 Example Format 1

3.4 Example Table Format 2

Note: The symbols d and v would need to be defined before being used in the table.

3.5 Unit format

There are two formats used for writing units. You will see both methods used in textbooks. Choose whichever format you are happiest with and stick to that format throughout your report, don't mix the two.

1. brackets:
   Examples: time (seconds) ; time (s) ; t (s)
2. forward slash:
   Examples: time /seconds ; time /s ; t /s

Writing units in brackets is probably the best choice. The brackets show up clearly and there is less confusion when writing a unit such as metres per second; i.e. (m/s) compared with /m/s.

If you are not sure which units to use then consider using the S.I. (International Standard) unit. There are also multiples of the basic units.

The Reference section has more detail about units:
Multiples : S.I. Units.

3.6 Time units

When writing time values it is important to separate the numbers correctly. Hours minutes and seconds are separated by a colon "3:21". Fractions of a second are separated by the normal decimal point "4.56".

When taking measurements from a stopwatch it is best to write down the time as it is shown on the watch. i.e. in minutes and seconds. If you make a mistake in converting this to just seconds you can always go back and double check your arithmetic; BUT if you do the conversion in your head and don't write down the raw value you have no way of carrying out this check.

For short time intervals of just a few minutes you are probably better off working in seconds rather than minutes. Using a combination of minutes and seconds can sometimes make it more difficult to work out the scales on your graphs.

example	unit	unit in words	meaning
1.50	(s)	seconds only	one and a half seconds
1.50	(min)	minutes only	one and a half minutes, or 90 seconds
1:50	(min:s)	minutes and seconds	one minute and fifty seconds
2:21.5	(min:s)	minutes and seconds	two minutes and twenty one and a half seconds
1.1	(h)	hours only	one hour and six minutes
1:10	(h:min)	hours and minutes	one hour and ten minutes
3:40:05.00	(h:m:s)	hours, minutes and seconds	3 hours, 40 minutes, 5 seconds
more time examples

If you are just writing a time in minutes it is probably best to write the unit as (min) rather than (m) which is the usual unit for length in metres. For combined units it should be clear from the context that (h:m:s) refers to hours, minutes and seconds.

3.7 Calculating an Average

The average of a set of values is equal to the sum of the values divided by the number of values. This is often called the mean value.

Example from the table above for the distance of 99.6 cm Repeated values are 3.5, 3.5, and 3.7.
sum of values	3.5+3.5+3.7 = 10.7
number of values	3
average	10.7 ÷ 3 = 3.5666666
This answer is best rounded up to 3.57

If you have a scientific calculator then it will have a statistical mode that can be used for calculating averages. Similarly, if you are using a spreadsheet for entering your data there are built in functions to calculate average values. This can save quite a bit of time if you have pages of data to analyse.

3.8 Accuracy

All numbers must be recorded to the precision you measured them to, including trailing zeros; e.g. if you are measuring length with a ruler you can measure to the nearest millimetre.
 60 mm or 6.0 cm or .060 m are all correct, but
 60.0 mm, 6 cm or .06 m all show the wrong precision.

When performing calculations using your raw results do not perform any rounding up or down until you have the finished answer. If you round up answers at each stage of the calculation you will loose accuracy.

It is best to use the same unit for all your measurements. Sometimes you may change scale when taking measurements. For example if you are using a digital ammeter you may change from an amp scale to a milliamp scale. It is important that you are consistent in the way you present your results. Either all the results should be presented in Amps, or all in milliamps, not both.

More examples, for the numerate...

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