Transcript Variables - Sackville School

Categoric Variables
At “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 Variables
Wendy collects balls,
She has lots of them
What is a categoric variable for the balls?
Colour, with Values of Green Ball, Red Ball,
Blue Ball & White Ball
Some of you may have said “Size” as a categoric
variable for the balls, this is a type of categoric
variable:
Ordered Variable
Ordered 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 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 boy
Without
legal issues
Continuous 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 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 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 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 facewash 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
0.6
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
1
2
3
4
5
9
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
Two of Joe’s results do not fit the pattern. Which results are they?
What was the independent variable in Joe's test?
Was the independent variable continuous, discrete or categoric?
What was the dependent variable?
Is the dependent variable continuous, discrete or categoric?
Write down one thing that Joe had to keep the same to make his test fair.
Random Error
This is where your data is generally quite
accurate apart from some results which are
randomly spread around the true value
120
Temperature
100
80
Random
Errors
60
40
20
0
0
2
4
6
Time
8
10
12
Systematic 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 1
2.05m
Measurement 2 Measurement 3
2.04m
2.06m
As you can see there is a systematic error of 5cm
Average
Measurement
2.05m
Zero Error
Last 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 & Precision
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.
Mass
(kg)
1
2
3
Average
14.50
14.49
14.50
14.50
These results also show good precision as there is
very little spread of results.
Calibration
Back 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 & 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
Evidence
Evidence is simply data which
can be used to make a
judgement as to what is
happening
Fair Test
This is a test where there is only one
independent variable which effects the
dependant variable
Validity & Reliability
Valid measurements are measurements which
give appropriate data.
Reliable data is data whose validity has
been evaluated