IBDP Biology · Internal Assessment
Measurement Uncertainty Helper
Find the uncertainty to report for your equipment, or calculate the uncertainty in a solution's concentration — with a source to cite and guidance on whether it actually affects your conclusion.
What do you need the uncertainty for?
Choose your equipment
Can't find it? Pick Other / not listed and the tool will work out the uncertainty from the equipment's resolution.
Uncertainty is taken as half the smallest division.
Uncertainty is taken as ±1 in the last displayed digit (use the manufacturer's stated tolerance if it is printed on the device or its manual).
Your measurement
Enter your reading in the unit shown — the uncertainty figure only matches if both are in the same unit.
This sensor's accuracy is quoted as a percentage of full scale, so the full-scale value is needed.
The colorimeter has no published accuracy figure, so use the smallest change its display shows (its last digit — often 0.001 A) or the standard deviation of your repeat readings. Enter it in whichever unit you selected above — if you are working in % transmittance, the tool converts it to absorbance for you.
Your repeat trials (strongly recommended)
You have almost certainly already calculated a mean and a standard deviation. Put them in here and the tool will tell you which uncertainty you should actually be reporting — the manufacturer's figure or the spread of your own data.
Use the SD for one condition (one point on your graph), not the SD of every value you collected. Leave blank if you only took single readings.
What are you investigating?
This changes whether the uncertainty really affects your conclusion.
Does it matter? (optional, but this is where the marks are)
e.g. if your control gave a mean rate of 1.20 and your treatment gave 1.75, the difference is 0.55. The tool will judge whether your uncertainty is small enough for that difference to be real — which is exactly the comment IB examiners are looking for.
How did you make the solution?
What this tool covers
Every measurement you record carries an uncertainty, and an IB Biology internal assessment has to state it and say whether it mattered. This tool gives you two figures for any reading: the absolute uncertainty (a ± in the measured units, which goes in your table headings and on your error bars) and the percentage uncertainty (the same thing as a fraction of the reading, which is what you add together when quantities are multiplied or divided). It then compares that figure against the standard deviation of your repeat trials, so you report whichever is larger, and against the difference between the groups you are comparing, so you can say whether the uncertainty was large enough to threaten your conclusion.
Vernier Go Direct sensors
Manufacturer accuracy figures, with a link to the Vernier product page so you can check them and cite the source: Go Direct O₂ Gas Sensor (GDX-O2), CO₂ Gas Sensor (GDX-CO2), pH Sensor (GDX-PH), Glass-Body pH Sensor (GDX-GPH), Optical Dissolved Oxygen Probe (GDX-ODO), Conductivity Probe (GDX-CON), Platinum-Cell Conductivity Probe (GDX-CONPT), Gas Pressure Sensor (GDX-GP), Temperature Probe (GDX-TMP), Wide-Range Temperature Probe (GDX-WRT), Colorimeter (GDX-COL), SpectroVis Plus Spectrophotometer (GDX-SVISPL) and UV-VIS Spectrophotometer (GDX-UVVIS).
Glassware and general lab equipment
Typical tolerances and reading uncertainties for measuring cylinders (10, 25, 50, 100 and 250 cm³), Class B volumetric flasks (100, 250 and 1000 cm³), Class B bulb pipettes (10 and 25 cm³), graduated pipettes, adjustable micropipettes, 50 cm³ burettes — separately for a single reading and for a titre, where the two readings double the uncertainty — two- and three-decimal-place electronic balances, thermometers, rulers and stopwatches. Anything not listed can be worked out from the smallest scale division or the last digit of a digital display.
Solutions you prepared
The uncertainty in a concentration made up from a mass of solid and a volume of solvent, or by diluting a stock solution, using the rule that percentage uncertainties add when quantities are multiplied or divided. It also checks whether your concentrations were far enough apart to be told apart given that uncertainty.
Things students commonly get wrong
- Quoting a stopwatch's ±0.01 s display resolution when hand-timing is really worth about ±0.2 s.
- Using the uncertainty of a single burette reading for a titre, which is the difference of two readings.
- Forgetting that weighing by difference means the balance is read twice, so its uncertainty doubles.
- Converting a pH or a Celsius temperature into a percentage uncertainty, which is meaningless on a logarithmic scale or one with an arbitrary zero.
- Reading a "% of full scale" specification as though it were a percentage of the reading.
- Reporting the manufacturer's figure when the spread of their own repeat trials is far larger.