Scientific Method

Coursework was introduced to science syllabuses in the early 1990s, attempting to address the concern that some students are not very good at exams.  Whole investigations were introduced to GCSE courses which gained a bad reputation because there were many vague criteria that were officiously applied to assess success. Whole investigations attempted to address some of the process of science, but they depended too much on paperwork and stifled creativity.

How Science Works is an umbrella term for getting students to think about the process of scientific investigation.  Many of the most significant scientific discoveries are the result of meticulous work and record keeping.  They have been accepted because not only are the results reproducible, but have been collected using methods that are considered to be reliable.

These notes will look at many common terms that are used in the laboratory.

# Variables

These are the items that are being studied in a scientific investigation.  In physics, the variables are almost always quantitative, i.e. have a value that is a number.

• Categoric variables are those that are described by words, for example, ants, bumble bees, butterflies.  There is no cross-over between the different classes.  You can’t have a half-way version of an ant and a bumble bee.

• Ordered variables are not numerical but are in a ranked order, for example, thin, medium, thick.

• Discrete variables are those that are in whole numbers.  You can have 1, 2, 3, …, but you cannot have 1.2 or 3.5.

• Continuous variables have a numerical value of any value, e.g. 3.64 W.

These variables are used in three ways in experiments:

• The independent variable is the one that we change (e.g. by changing the length of resistance wire);

• The dependent variable changes in response (e.g. the resistance increases).

• The control variable, which is kept constant to make sure that it’s a fair test.  If you were measuring temperature rise with the power of a heater, you would keep the start temperature the same, use the same volume of water, etc.

Any type of variable can be the independent or dependent variable.

# Taking Measurements

When you read your instruments, you are recording data.  The data that you record are then processed in a certain way to get:

• A graph;

• Processed data items, (e.g. resistance from voltage and current);

• Or both.

 Note that the word data is a plural word.  The singular, datum, is used in the context of a reference point from which measurements are made.   A single measurement is best referred to as a data item.

Data are made reliable by taking repeat readings.  Whenever you take a set of repeat readings, there is always going to be a certain amount of variation.  A free-fall experiment involving dropping a ball bearing through a viscous (gooey) liquid may give these results:

10.1 s, 10.2 s, 9.9 s, 10.0 s; 10.3 s; 10.6 s

There is variation because there are reaction times in operating the stopwatch.

The range of the results can be worked out:

Range = maximum value – minimum value

 Question 1 What is the range of the data above?

To get a value that we can plot on a graph, we need to take an average (or mean):

Average = (Sum of the readings) ÷ total number

 Question 2 What is the average of the data above?

Notice from your answers that the average is not necessarily the midpoint of the range.

You can also test for the reliability of your data by checking the results obtained by other students in your class.  If they are close, you can be confident that your data are reliable.

# Accuracy and Precision

When you take a measurement, you want the measurement to be accurate and precise.  However it is important to understand the difference between the two words:

• Accurate measurements are close to the true value;

• Precise measurements arise from the smallest scale reading that the instrument can give.  A digital voltmeter can give a precision of 0.001 V (the minimum reading that it can take).

If the voltmeter is not properly calibrated, it may be precise, but not accurate.

When measuring length with a metre ruler, you can measure to a precision of ± 1 mm.

An electronic thermometer that can read to ± 0.1 oC is more precise than an alcohol thermometer that reads to ± 0.5 oC.

However there is more to precision than the above:

• A precise instrument gives consistent readings when taking the same measurements.  For example:

• A beaker is weighed 3 times on balance A.  The readings are 73 g, 77 g, and 71 g.  The range is 77 – 71 = 6 g.

• The same beaker is weighed on balance B.  The readings are 75, 73, and 74 g.

Question 3

What is the range for balance B?  Which instrument is more precise?

Question 4

Three students are asked to determine the capacity of a box for storing ball bearings. Each of them uses a different ruler and only one takes care doing it. Here are their results:

 Name Width (cm) Height (cm) Depth (cm) Capacity (cm³) Andy 10 9 7 Bill 11 8 6.5 Cass 10.8 7.9 6.4

Calculate the capacity for each student.

The true value is 545 cm3.  Which is the most accurate result?

# Reliable and valid evidence

Evidence is data that are considered to be relevant to the investigation.  When investigating resistivity, the lengths of the resistance wire are relevant data; they are evidence.  The colours of the connecting wire or the manufacturers of the meters are not.  They are not evidence.

We want our evidence to be reliable:

• Reliable evidence stems from data that you can trust.

• If someone else did the same experiment, they would get the same result.

The evidence must be valid as well.  Valid evidence is reliable and relevant.  For example:

• Measuring the extension of a spring to find the force pulling on it.  This is relevant, so the evidence is valid, provided that the data are reliable.

• Measuring the volume of a resistor to investigate the resistance.  This is not valid evidence, as the volume is not relevant to the resistance.

You can check the validity of your data by using secondary evidence, e.g. someone who has done the experiment before, and observed the same things.