Categoric VariablesCategoric VariablesAt “Mad Bob’s Random Zoo of Insanity” Bob has a rather strange group of animals. If one of the animals was to fall down a old abandoned mine shaft the variable to which animal fell (the categoric variable) is one of 3 categories (the values): Cow, Tiger or Rabbit

Categoric VariablesCategoric Variables

Wendy collects balls,Wendy collects balls,

What is a categoric variable for the balls?

Colour, with Values of Green Ball, Red Ball, Blue Ball & White Ball

She has lots of them

Some of you may have said “Size” as a categoric variable for the balls, this is a type of categoric variable:

Ordered Variable

Ordered VariableOrdered Variable

This is a type of categoric variable where the values can be ordered or ranked.

The ordered variable for the balls could be size, where the values would be large, medium and small

Discrete VariablesDiscrete Variables

These are variables where the values can only be whole numbers. For example a discrete variable for the school could be the number of boys in a class. This must be a whole number (an integer)

You cant have half a boyWithout legal issues

Continuous VariablesContinuous Variables

This is a variable which constantly varies over time and data should be taken either at a particular moment or over a period of time

For example how the temperature of a room changes over a day

Independent VariablesIndependent Variables

This is what you decide you are going to change in an investigation to see what effect it has.

For example if you are investigating how the mass of a toy car effects its speed down a ramp the independent variable is the cars mass

Dependant VariablesDependant Variables

This is a variable which changes as you change the independent variable. It is the variable that you measure.

For example if we are investigating how the steepness of a slope effects a toy car’s speed, then the speed is the dependant variable

Control VariablesControl Variables

This is something which may effect your results but isnt what you are changing in the investigation (it isnt the independent variable)

Control variables should be kept constant to ensure a fair test

For example if you were seeing how the height dropped from effects an egg hitting the floor your control variables could be the type of egg and the floor surface

Natalie is comparing an ‘off the shelf’ antibacterial face-wash with a antibacterial hand wash that is used in hospitals.She placed three discs, each loaded with one drop of the agent, onto an agar plate that had been inoculated with bacteria.

She repeated the investigation three times with each antibacterial agent.Her results are shown in the tables below.

Face-wash Radius (cm)

Plate 1 Plate 2 Plate 3

Disc 1 0.7 0.7 0.6

Disc 2 0.7 0.8 0.5

Disc 3 0.6 0.8 0.8

Average 0.7 0.8 0.60.6

Hospital hand wash

Radius (cm)

Plate 1 Plate 2 Plate 3

Disc 1 1.6 1.5 1.2

Disc 2 1. 5 1.8 1.5

Disc 3 1.2 1.7 1.3

Average 1.4 1.7 1.3

1 What is the independent variable for this investigation? 2 Is the independent variable continuous, discrete or Categoric? 3 What is the dependent variable? 4 Is the dependent variable continuous, discrete or Categoric?

Joe investigated the power produced by solar cells with different areas. He used card to block out some of the cells on a solar buggy and then used a speed sensor to measure the speed that the buggy could reach.Joe's results are shown in Table B below. You will notice that he carried out each test three times.

Number of cells left uncovered

Speed of buggy (m/s)

1st test 2nd test 3rd test

1 0.29 0.31 0.28

2 0.59 0.58 0.63

3 0.92 0.79 0.87

4 1.11 1.22 1.20

5 1.52 1.48 1.51

6 1.77 1.81 1.79

1 Two of Joe’s results do not fit the pattern. Which results are they?2 What was the independent variable in Joe's test? 3 Was the independent variable continuous, discrete or categoric?4 What was the dependent variable?5 Is the dependent variable continuous, discrete or categoric?9 Write down one thing that Joe had to keep the same to make his test fair.

Random ErrorRandom Error

This is where your data is generally quite accurate apart from some results which are randomly spread around the true value

0

20

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60

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100

120

0 2 4 6 8 10 12

Time

Tem

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atu

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Random Errors

Systematic ErrorsSystematic Errors

This is where a set of data has good precision (very little spread of results) around an incorrect mean value

The table below shows a systematic error for measuring a 2m long table

Measurement Measurement 11

Measurement Measurement 22

Measurement Measurement 33

Average Average MeasurementMeasurement

2.05m2.05m 2.04m2.04m 2.06m2.06m 2.05m2.05m

As you can see there is a systematic error of 5cm

Zero ErrorZero ErrorLast lesson you used a stick to measure a room or the playground. A problem with this is that the zero cm mark was probably not exactly on the end of the stick. This is called a zero error. The extra bit of stick will then be counted as part of the length making the results inaccurate.

It can also be seen when meters like ammeters or voltmeters do not stop exactly on zero.

Accuracy & PrecisionAccuracy & Precision

11 22 33 AverageAverage

Mass Mass (kg)(kg)

14.5014.50 14.4914.49 14.5014.50 14.5014.50

Accuracy is when the dependant variable has repeated values which are close to the true value.

Below is a table of accurate result for the mass of a rucksack whose true mass is 14.50kg.

These results also show good precision as there is very little spread of results.

CalibrationCalibrationBack in ye olde days sailors used to use a rope tied to a log to measure boat speed. They would tie knots in the rope a set distance apart and time how many knots passed through there hands in a certain amount of time.

Putting the knots in the rope at a set distance apart calibrated the rope to measure speed

Data & DatumData & Datum

Psycho Sarah likes kicking rabbits, she kicks one rabbit 2 metres

This single measurement is called a datum

She kicks a 2nd rabbit 1 metre

These 2 measurements together are called data

EvidenceEvidence

Evidence is simply data which can be used to make a judgement as to what is happening

Fair TestFair Test

This is a test where there is only one independent variable which effects the

dependant variable

Validity & ReliabilityValidity & Reliability

Valid measurements are measurements which give appropriate data.

Reliable data is data whose validity has been evaluated