PSA and ISA

Practical and Investigative Skills Assessments

This is the coursework element of your exam. It's worth 25 % of your total mark. Get good marks in your centre assessed work and you will be in a good position to get a good grade.

There are two aspects:

The Practical Skills Assessment.
The Investigative Skills Assessment.

The Practical Skills Assessment

Here you do a practical set by your teacher who observes how you tackle the task, and gives you a mark out of 6.

For 2 marks, you need to work safely. You may be given some help to set up the experiment and take down measurements.
For 4 marks, you need to work in an organised way as well as safely. You proceed without help and use the apparatus skilfully. You record your measurements in a logical way in neat tables.
For 6 marks, you need not only to work confidently, safely, and without help, but also some of the apparatus should be more challenging. Your data are recorded on tables you have prepared yourself. You should take accurate readings, to a level of precision appropriate to the experiment.
You can score intermediate marks, like 3, or 5, if you don't meet all the criteria for 4 marks or 6 marks.

PSA experiments are done throughout the year, and your best marks will be submitted to the Board.

The Investigative Skills Assessment

Key Words to Learn

Variable: A part of the experiment that you could change, e.g. the size of the beaker or the amount of substance.

Dependent Variable: The variable you will measure, e.g. change in temperature or the time something takes. This variable is plotted on the vertical axis of a graph.

Independent variable: The variable you deliberately changed, e.g. concentration of acid or size of current. This variable is plotted on the horizontal axis of the graph.

Fair test: Changing only 1 variable and keeping all the other variables the same. This means you are only testing the effects of one variable, e.g. keep the volume of water the same.

Precision: How accurate the measuring device is, e.g. do the weighing scales go to 2 decimal places? 12.6g is less precise than 12.57g or 82ºC is less accurate than 82.39ºC. Precision can be improved by using better equipment, or a better range or scale.

Accuracy: How correctly the operator reads the equipment. Saying it’s about a minute is not as accurate as saying 62 seconds. You can improve accuracy by checking your reading with someone else. Take care writing down your results in your table, e.g. writing 3.62 instead of 3.26.

Reliability: This is how sure you are that your data are correct. You can improve reliability by repeating the experiment and taking an average of the results.

Anomalous results: These are results that do not fit the pattern of all the other results.

In this assessment, you are set a task in which you record your own data in an experiment. You then have to process these data, by putting them into a table, and plotting an appropriate graph. Then there is an examination (done under examination conditions). In Section A, you answer questions about your data. In Section B, you are given data, and you are asked to interpret them.

The marks are:

6 marks for your data and graph;
Section A carries 10 - 14 marks (not including your data);
Section B carries 14 - 20 marks;
Whatever the marks given to each bit, the total adds up to 34 marks.

These notes are here to help you understand some of the key words and processes that you need to carry out to score well.

1. The Planning

You may be asked to plan an experiment. You need to think carefully about what you have to do:

You will need to select what you are going to change yourself (the independent variable).
Which factor will change as a result of your changes (the dependent variable)?
What are you going to keep the same to make it a fair test?
How are you going to make sure that the data are reliable (repeat the experiment three times and take an average)?
What kind of numbers will you get? What ranges will you use on the measuring instruments? (It's no good using a metre rule to measure the thickness of a wire.)
How will you keep the data accurate?
What precision will you use? (1 to 2 decimal places is quite enough; don't write the huge numbers of decimal places you calculator gives.)
You are expected to work safely, and state how you do so in your plan.
You will need to state how you are going to present your data.
You may be asked to make a prediction, i.e. what you think will happen. You can use what you have learned in previous lessons to help you.

You are expected to do this on your own.

2. The Practical Work.

It is important that you take the practical work seriously.

Although sample data can be given, you will not score any marks for your practical work, although you will gain credit for correct answers in Section A.

You will be given some instructions that you need to read carefully. You then need to do the experiment safely and responsibly. You may work in groups but every student must have their own set of results. Excuses like "Wayne had them, and he's not in today" are simply not acceptable.

It doesn't matter how roughly you taken them down, as long as you record your measurements accurately. However you must have the results put into a neat table:

The table is boxed in with lines drawn with a ruler, please;
The quantities are placed as column headers;
The units are put in with the quantities in the header.
Designing your table should be done under examination conditions.

Here's what NOT to do:

These data are neither use nor ornament. The following picture shows a more organised approach:

Notice that:

The columns are headed with the quantities (time, temperature);
The table has been boxed in;
Repeat readings have been taken, and the average (to the nearest whole number) has been worked out;
There is NO problem if you make a mistake. Cross it out neatly.
There is a wrong result. Ignore it and take the average of the two readings.

We take repeats to reduce the uncertainty. On its own, the set of data with the wrong result might lead us to the wrong conclusion. If we have other data to compare it with, we can be confident that that data point was wrong. So we can ignore it.

3. The Graph.

Why do we have to draw a graph? The answer is that a table of numbers is not all that helpful. Look at the table of numbers above and the only thing you can say is that the temperature increases with the time. The graph gives us a picture of what the data are telling us.

What sort of graph should we draw? It all depends on what you are investigating. We need to know what kind of variables we are dealing with. Variables are the quantities that change in the experiment:

Categoric variables are those that cannot be turned into numbers. In a biology ISA, we might be investigating numbers of small animals in a square metre, e.g. woodlice, earthworms, springtails, ground beetles, etc. In this case we would need to draw a bar chart.
Discrete variables are those that go up in whole numbers, for example, layers of newspaper in an insulation experiment. You can have 0, 1, 2, 3... You cannot have 1¼ layers of newspaper.
Continuous variables have any number you like, not just whole numbers, e.g. 3.6 V, 1.25 s.

If our variables are continuous, we would mostly plot a line graph.

In our table is the independent variable, which means the variable that we changed; in this case we decided that we would see what happened when the time changed. Time was our independent variable.

The dependent variable is the quantity that changes because of the changes we have made. In this case, the temperature is going up as the time is going up.

Normally we plot the independent variable on the horizontal axis (the x-axis) and the dependent variable on the vertical axis (the y-axis).

Here is what NOT to do:

An extreme case perhaps, but such pitiful drivel is not unknown. I am sure that you can tell what is wrong here.

A correctly drawn line graph is shown here:

Notice that:

Each axis this labelled, with the units.
The scales are calibrated (the numbers put in) in even steps.
The graph is drawn in pencil.
The data points are plotted as crosses.
A line of best fit is drawn through the points.
A title tells you what the graph is about.
A point is plotted when time = 0.

It can be difficult to decide where the line of best fit goes; you just need to do what you think is best. However there may be things that will help you:

If two quantities are linked by a simple formula, like V = IR, then if the voltage is 0, the current is 0. So the origin is a good point to start.
The graph above does not start at the origin, because the temperature was 16^oC when we started the stopwatch. Don't force your line of best fit through the origin in this case.
Sometimes the graph may show a trend that is a curve. Don't force a straight line through what is clearly a curve.
Sometimes data points are well away from the rest of the points. These are anomalous results that don't fit in. So don't include them.

Tables and graphs must be done in class, not taken home. They must be your own work, not done with somebody else.

4. The Exam

You will have to answer questions on the ISA under examination conditions. This means not only that you must work on your own and are not allowed to talk to others, but also that you have to follow the rules of all the examination boards:

You must not take into the exam room any mobile telephone. If you do, you will be reported to the Board and you may be disqualified, even if it's turned off.
You must not take any material that could give you an unfair advantage. You will need to take your bags and coats off and leave them at the back of the room, or as directed by your teachers.
You may not ask for, nor will you be given, any help in interpreting what a question means.
You are expected to provide your own calculator.

The best advice for preparing for the exam is to revise the subject for which you did the experiment. If you are not sure, ask your teacher.

Section A

You will be asked questions about the data which you had collected and your graph.

You will be asked how you made the experiment a fair test, i.e. what variables did you keep the same throughout the experiment? For example, when you look at the experiment above, you would keep the amount of oil the same, and the same container.

You will be asked if there were any anomalous results. These are results that don't fit into the rest. If you can, try to say why a result was anomalous, e.g. the thermometer was right next to the heater. If there were no anomalous data, say so.

You may get asked about accuracy and precision:

Accuracy means whether we have read the instrument correctly.
Precision is how many decimal places a particular instrument reads.

Think of a dartboard:

– If you aim for the bulls-eye and get 3 in the treble 20, it’s good accuracy, but not precise.

–1 in the bulls-eye and two somewhere else, it’s precise, but your three darts aren’t accurate.

By taking repeat readings, you are improving the reliability of your experiment. If the data are not reliable, you cannot really support your prediction (what you think will happen). As far as a school laboratory is concerned repeated data within 10 % are pretty reliable. A striker who puts a football 1 cm off course in a penalty will hit the post. He is not reliable.

You may be asked about uncertainty (often called errors):

Systematic uncertainty comes the apparatus itself. You cannot change the measuring instruments, but you may be able to reduce heat loss by insulating the beaker of oil, and putting a lid on it.
Random uncertainty, that usually arises from the limitations of the people doing the experiment. For example, was the thermometer read accurately? Was the reading taken at precisely 80 seconds?

You will be asked about the patterns in your data:

You need to identify a pattern (“when the voltage doubled, the current doubled too”).
Exclude anomalous results.
Write your conclusion (what you understand by the results) e.g. the voltage is proportional to the current.
You may need to carry out further investigations to make your conclusion firmer. These are not more of the same. You might change one of the other variables that you kept the same to make it a fair test.

You could be asked about the limitations of your experiment:

•Sometimes it’s difficult to collect enough evidence to answer a question properly.

•Some questions cannot be answered by science alone.

•Sometimes scientists cannot carry on with experiments because they are dangerous, anti-social, or unethical.

Section B

This section asks you to interpret data in a new and unfamiliar context. If you have reviewed the topic that the practical work is involved with, you will have more confidence. It is not possible to give specific advice for specific questions, but here are some points to look out for:

If there are numerical data, there will be one data item that is anomalous.
You will be asked to identify a trend from the numbers.
There will be other questions, particularly about the impact of the science on society.
Never leave blanks; always attempt something for the answer. You might get it completely wrong, but at least you have had a go. You might hit upon the right answer without realising it. Blanks can only get zero. The examiners love blanks; they get paid per script, and all they have to do is put a read line through the blank paper to show that they have seen everything. Make the examiner work.

If you want to know more, click on the Physics for You website button. There is also a PowerPoint on How Science Works. Use the HSW button to see it.