Making and recording measurements
The term data (singular = datum, or data value or variate) refers to
measurements of a particular characteristic, or variable, classified as:
- Quantitative: where the individual values are described on a numerical scale
which may be either
- continuous, taking any value on the measurement
scale, or
- discontinuous (or discrete),
where only integer values are
possible. Many of the variables measured in chemistry are continuous and
quantitative, e.g. weight, temperature, time, amount of product formed in
an enzyme reaction.
- Ranked: where the data values can be listed in order of magnitude.
Where such data are given numbered ranks, they are sometimes called
'semi-quantitative data'. Note that such ranks cannot be treated as 'real'
numbers and they should not be added, averaged, etc.
- Qualitative: where individual values are assigned to a descriptive category,
e.g. the detection of the presence or absence of a chemical by a colour test
or precipitate.
Variables may be independent or dependent. Usually, the variable under the
control of the experimenter (e.g. time, reagent concentration, pH, etc.) is the
independent variable, while the variable being measured is the dependent
variable. Sometimes, it is inappropriate to describe variables in this
way, and they are often referred to as interdependent. Another group of
values, often termed derived (or computed) data, are calculated from two or
more individual measurements, and these include ratios, percentages and
rates.
