Units, axes and other signs of civilisation
A number without a unit is often useless.
Five metres, five millimetres and five degrees describe different situations. A table with no units can look convincing right up to the moment someone uses it.
Technical discipline requires that you state:
- units of measurement;
- the coordinate system;
- the scale;
- the data source;
- the date;
- the version;
- the symbols used;
- the assumptions;
- the range of applicability.
A graph needs labelled axes. A drawing needs a legend. A calculation needs its inputs and formulas. A document needs a status and a version.
These elements look redundant to the author, because he remembers the context. The document usually outlives that memory.
Assumptions shouldn’t be banned outright. Without them you can’t design, forecast or decide on incomplete information.
An assumption should be:
- explicit;
- justified;
- testable;
- conservative enough where there is risk;
- revised once data arrives.
“I don’t know all the conditions, so I’m not entitled to propose a solution” is sometimes sensible. Sometimes it becomes a way of never deciding anything.
It is more accurate to say:
Given these inputs and these assumptions, this option looks best supported. If a condition changes, the decision has to be revisited.
Technical honesty is not the absence of uncertainty. It is keeping the uncertainty out from between the lines.
The most expensive numbers are the ones whose unit is implied. “A cup of flour” weighs somewhere between a hundred and twenty and a hundred and eighty grams, and the dough reports the difference honestly. A clearance of “0.5” with no millimetres stated arrives on site one day as a part that has to be remade at your own expense. In both cases the author knew perfectly well what he meant. That knowledge simply didn’t travel with the document.
A document lives longer than its author’s memory and almost always reaches someone who can no longer ask him: he is at another company, in another city, or simply doesn’t remember. A labelled axis isn’t pedantry, it is a way of not having to attend every future reading in person.