Accuracy and Precision

If a person goes into a restaurant and orders a cup of coffee, how much coffee will be given to the person? Will the coffee arrive in a large, earthenware mug or a delicate, china cup? Do all coffee cups hold a cup of coffee? If, on the other hand, another person Is following a recipe that called for a cup of coffee, how much coffee will that person use?

Will there be a difference in how the coffee is measured in these two situations? Recording numerical data is an important part of scientific research. The reliability of these data can Influence the conclusions drawn from the experiment. Although “accuracy and “precision” are used interchangeably in common speech, in scientific language, they mean two different things. The “true value” of any number is a philosophical idea which we take as a given/known thing; for example, scientists say that exactly 100. 0000 ml of water weigh exactly 100. 0000 g at 40 C . An “error” in data is the numerical deference between the measured value and the true value. An “accurate” result is one that agrees 100 ml of water, a weight of 100. 001 g is more accurate than 100. 009 g, and that is more accurate than 100. 01 g. “Precision,” on the other hand, refers to agreement among a group of data, but says nothing about their relationship to the true value. Three measurements of 100. 009, 100. 008, and 100. 007 g might be more precise than three measurements of 100. 009, 100. 002, and 99. 995 g, and yet may not be more accurate.

In the above example, which of these methods of measuring coffee is the most accurate? If a measuring cup is used, will that always measure exactly one cup of coffee? Why or why not? What factor(s) could be sources of error in the user’s measurement? Which of these methods of measuring coffee would be the most precise? Why? There is a variety of glassware here in the Biology Lab – beakers, graduated cylinders, Erlenmeyer flasks, volumetric flasks – that could be used for a lab exercise in which students would be required to measure 100 ml of distilled water (dhow).

Because these various types of lab glassware are designed for different purposes, their accuracy and precision vary. Certain types of glassware are manufactured with greater precision than other types and/or yield more accurate agreement of volume. Knowledge of the relative accuracy and/or precision of the various types of glassware can aid in determining the appropriateness of a piece of glassware for a desired use. For example, if a student needs several identical 100 ml samples, which measuring utensil should be chosen? Why?

When a scientist comes up with an answer to a question like the preceding one that might be right yet needs to be tested to see if it is true, this is called a hypothesis (hypo under, beneath; thesis = an arranging). Any testable answer to the previous question such as, “l think that the  glassware is more Once a scientist has formed an hypothesis, it is then necessary to figure out how that hypothesis can be tested. The scientist would need to decide what to do (procedure/methods) and what data are appropriate to gather to uphold or disprove the hypothesis.

At times, scientists may end up gathering “negative” data that actually disprove their hypotheses. For this glassware, what could be done – what steps could be followed – to find out if the _ glassware really is the most precise/accurate? Is it enough to use one piece of glassware or should several kinds/styles be tried? Is it enough to take one reading on each piece of glassware or should several tests/trials be reformed on each piece? If a person places a desired amount of water into a piece of glassware, how will that person know if the container is correctly filled?

How will (s)he know the container is filled the same amount every time? When viewed from the side, the surface of the water in a transparent glass container is a characteristic shape that is a clue to solving this dilemma. Because of water’s affinity for glass (glass is hydrophilic, hydro = water, Philip = brotherly love), the edges of the water’s surface will creep up the walls of the container slightly. Especially in small-diameter glassware, the surface of the water is, thus, tactically curved. This curved surface of the water is called a meniscus (menisci = a crescent).