Sources of Error
Read the following information and then take the quiz that is linked at the bottom of this page.
Mistakes vs Sources of Error
Experiments never turn out exactly as we predict. There are different reasons for this. Sometimes, we make mistakes:
We measure wrong
We write down the wrong measurement
We calculate wrong
We spill something
We forget to zero the balance
We calibrate wrong
Mistakes can have different labels: blunders, goofs, mishaps, oversights, etc.. In general, mistakes are avoidable; they can be identified and fixed.
On the other hand, our results can be thrown off by unavoidable factors. These factors can be minimized, but never fully eliminated. They are known as sources of error. Sources of error are not the same as mistakes.
Sources of Error Include...
Subjectivity
Whenever data collection depends on judgment. Our senses are imperfect, and their imperfections are further compounded by our judgment.
Assumptions
Every experiment involves assumptions. While these assumptions never fully represent reality, we allow for them because the extent to which they are inaccurate is usually minimal.
Confounding Variables
When you carry out an experiment, you are trying to see the effect of one variable on another. Other variables that can have an undesirable effect on the outcome are known as confounding variables. Although we can account for confounding variables to an extent, we can never control them completely.
Chemical Purity
The chemicals that we use are usually not 100% pure. Even if they are pure, they are prone to degradation over time, as in the rusting of metal by oxygen in the air. The presence of impurities can lead to inaccuracies in measurements (such as mass).
Instrumental Interference
Sometimes the act of taking a measurement can affect the reading itself.
Non-Representative Sampling
When we collect a series of samples, we assume that the samples represent a population. Although taking repeated measurements is a way to combat experimental variability, there is always the uncertainty that our samples are not representative of the whole.
Instrumental Limitations
Measuring instruments have a limit to their reliability. For example, the electronic balances in the general chemistry lab are not accurate past 1/1000th of a gram (0.001 g). In addition, the reliability of an instrument’s reading can drift over time. This tendency to drift necessitates the calibration of our instruments.
Calibration is a process in which an instrument is set to a known value. For example, the electronic balances are calibrated with a standard mass, a block of metal that is known to be 100.000 grams. Some instruments need to be calibrated periodically (once a year in the case of the balances) while others need frequent calibration (pH probe, turbidity probe, conductivity meter, and spectrophotometer). When you use an instrument that needs frequent calibration, it is necessary to constantly evaluate instrumental drift and calibrate/re-calibrate as needed.
You will be asked to evaluate sources of error in every experiment. You will need to think about how some of these could apply to that specific experiment. That is, it is not enough to say that a source of error is simply "subjectivity".
Sources of Error Quiz
Info
You will need to log in with your USM e-mail and password.
Feel free to refer back to this page as you complete the quiz.
To complete the quiz, you must answer all of the questions correctly. You will have unlimited attempts to answer each question.
References
West York Area High School. (2013). Sources of Experimental Error WYChem. YouTube. Retrieved December 13, 2023, from https://www.youtube.com/watch?v=ahraXWLN0ec.