• The meaning of ‘data’ and ‘evidence’,

• What is meant by ‘reliable’ evidence and ‘valid’ evidence,

• What is meant by ‘secondary’ evidence.

Learning Objectives

You should learn :

Definitions…data …

evidence …reliable …

valid……but what do they mean??

Definitions : data

‘data’ is a measurement, or measurements.

For example:

When you read a thermometer: the temperature readings are data.70oC, 68oC, 66oC, 64oC, ….

Strictly speaking, ‘data’ is plural.

So: 1 ‘datum’, 2 data, 3 data, ….

Definitions : evidence

‘evidence’ is data which is judged to be relevant.

For example:When you investigate cooling: the temperature data is relevant,so it is evidence.

The length of the thermometer

is not relevant,so it is not evidence.

Definitions : evidence

‘evidence’ is data which is judged to be relevant.

The evidence needs be:

• Reliable

and

• Valid.

Reliability

Reliable data is evidence you can trust.

If someone else did the same experiment,they would get the same result.Your evidence will be more reliable if you repeat your readings.For example…

Reliability For example:

3 students measure the time for 1 swing of a pendulum:

Discuss which method is the most reliable, and why.

• Jo measures 1 swing.• Emma measures 1 swing,

but 20 times, and calculates the average (mean) time.

• Jack measures 20 swings and divides the time by 20.

Physics for You page 359

Validity

Valid data is evidence that is reliable and which is relevant to the question being investigated.

Just being reliable evidence is not enough.

The evidence has to be relevant as well.For example…

Validity

Discuss which of the following is valid evidence:

Example 1Measuring the length of a magnet to decide its strength?

No, this is not valid evidence.

A long magnet can be strong or weak,

and a short magnet can be strong or weak.

N S

N S

Validity

Discuss which of the following is valid evidence:

Example 2

Yes, this is valid evidence.

Measuring the extension of a spring to find the force pulling on it?

Validity

Discuss which of the following is valid evidence:

Example 3Measuring the volume of a firework to find the energy in it?

No, this is not valid evidence.

A large or a small firework can have a lot of energy or very little energy,depending on the chemicals inside.

Secondary evidence is data collected by someone else.

Secondary evidence

You may find it in a book or on the internet

BUT

You should always check to see if it is reliable and valid.For example…

Secondary evidence is data collected by someone else.

Secondary evidence

Example 1

Some data on the pollution from a car is published by the car manufacturer.

Would you trust this evidence, without further data?

Secondary evidence is data collected by someone else.

Secondary evidence

Example 2

Some data on the radiation emitted from a mobile phone is published by the phone company.

Would you trust this evidence, without further data?

• The meaning of ‘data’ and ‘evidence’,

• What is meant by ‘reliable’ evidence and ‘valid’ evidence,

• What is meant by ‘secondary’ evidence.

Learning Outcomes

You should now understand: