Error Analysis

As a scientist you must make the best effort to avoid errors! Errors may be present in: - the design of the experiment. -the use of instruments. We will now discuss the sources of error.

Sources of error

Human error Concentration must be kept when doing an experiment. Read handouts in advance so you understand why you are doing the experiment and the principles behind it. Consider safety hazards before you begin. Organise your working bench space. Equipment must be free of contaminants. Data must be recorded carefully and neatly.

Measuring Liquids Liquid must be stirred before checking temperature Thermometer should not rest on bottom of flask Graduated cylinders should be read at lowest point of meniscus and at eye level. If you don’t know how to use an instrument, ask!

Liquids likely to froth e.g. yeast or protein solutions are difficult to measure; transfer slowly Suspensions e.g. yeast or cell cultures may sediment; mix well before transferring

Using syringes Make sure there are no air bubbles in syringes when measuring volumes. Expel liquids slowly and touch the end of the syringe on the vessel to remove any liquid stuck to the end

Measuring solids Never weigh anything directly onto a balance’s pan. This will contaminate it for other users. Use a weighing boat or slip of aluminium foil, or paper Make sure all of the sample is removed from weighing boat after weighing.

Measuring time Use a stop watch rather than a clock Make sure you know which buttons to press before the experiment starts!

Calibration error Electronic instruments (e.g. pH meter, balance, water bath, spectrophotometer) need to be checked periodically to ensure they are calibrated. If this is not done regularly or if it is done incorrectly it will increase error.

Random error Biological material is notably variable. Therefore it is important to have a large sample size. You should have a sample size of at least 5.

The Act of measuring When a measurement is taken this can affect the environment of the experiment. Example: A cold thermometer put in a test tube of warm water will increase the water temperature. Example: when the behavior of animals is being recorded the presence of the experimenter may influence them. Holding a beaker in your hand will increase the temperature of the liquid.

Uncontrolled variables What variables were not controlled? What effect might each of these uncontrolled variables have had on your data and conclusion?

Why bother? You might think that with all these sources of error experimental results are worthless. This is NOT TRUE, it is understood that experimental results are only estimates. What is expected of scientists is that they: make the best effort to avoid errors in their design of investigations and the use of instruments. are aware of the source of errors and appreciate their magnitude.

Degrees of Precision and Uncertainty in Data

For rulers and instruments with digital displays (like our balances) The degree of precision is plus or minus (±) the smallest division on the instrument. How would you record the mass on the balance at right? 14.8 grams +/- 0.1 gram

For most other instruments such as thermometers, graduated cylinders and pipettes The degree of precision is plus or minus (±) one half the smallest division. How would you record the volume in the cylinder on the right? 43.0 ml +/- 0.5 ml

A pipette whose smallest unit is 0.1 ml has an uncertainty of __________. How would you record a pipette measurement of 34.1 ml? +/ ml ml ± 0.05 Notice a zero is added to Now it matches the number of decimal places of the uncertainty. This is very important!

When you have data that is derived from 2 different variables you need to use the Upper and Lower Bounds Method for estimating uncertainty. I f you wanted to determine whether large insects absorb less insecticide per unit mass than smaller ones, you would have to measure the amount of insecticide they absorbed (in milliliters) and divide it by their mass (in grams) and you would end up with an estimate in milliliters/gram of insecticide absorbed. Since we are dealing with two different units (milliliters and grams) we must use the upper and lower bounds method. Insect #1: Absorbed 0.23 ml +/- 0.01ml with a body mass of 5.2 g +/- 0.1 g First, calculate the rate ignoring the uncertainties: 0.23ml/5.2g = 4.42 x ml/g Next, calculate the Upper bound: 0.24 ml / 5.1 g= 4.70 x ml/g Lastly, calculate the Lower bound: 0.22 ml / 5.3 g= 4.15 x ml/g Therefore, the Answer would be 4.42 x ml/g +/ x ml/g

Most Important Rule: Your measurements and your uncertainties must always agree. Example: Recording a value of 10.3 ml +/- 1 ml is incorrect. Why? Example: It is incorrect to record 10ml +/- 0.5 ml. Why? If the uncertainty is +/- 1, it must be written as 10 ml +/- 1 ml. It should be written as 10.0 ml +/- 0.5 ml. The number of significant figures should always be the same.