A single source of truth?

Lately in a chatroom for the International Society for Software Testing there has been some discussion about the idea of a “single source of truth”. I’m familiar with this in the sense of database design. Every piece of data is stored once and the design precludes the possibility of inconsistency, of alternative versions of the same data. That makes sense in this narrow context, but the discussion revealed that the phrase is now being used in a different sense. A single source of truth has been used to describe an oracle of oracles, an ultimate specification on which total reliance can be placed. The implications worry me, especially for financial systems, which is my background.

I’m not comfortable with a single source of truth, especially when it applies to things like bank balances, profit and loss figures, or indeed any non-trivial result of calculations. What might make more sense is to talk of a single statement of truth, and that statement could, and should, have multiple sources so the statement is transparent and can be validated. However, I still wouldn’t want to talk about truth in financial statements. For an insurance premium there are various different measures, which have different uses to different people at different times. When people start talking about a single, true, premium figure they are closing off their minds to reality and trying to redefine it to suit their limited vision.

All of these competing measures could be regarded as true in the right context, but there are other measures which are less defensible and which an expert would consider wrong, or misleading, in any context (eg lumping Insurance Premium Tax into the premium figure). That’s all quite aside from the question of whether these measures are accurate on their own terms.

A “single source of truth” reminds me of arguments I’d have with application designers. Sometimes the problem would be that they wanted to eliminate any redundancy in the design. That could make reconciliation and error detection much harder because the opportunities to spot errors would be reduced. If a calculation was wrong it might stay wrong because no-one would know. A different source of friction was the age old problem of analysts and designers determined to stick rigidly to the requirements without questioning them, or even really thinking about the implications. I suspect I was regarded as a pedantic nuisance, creating problems in places the designers were determined no problems could ever exist – or ever be visible.

Accounting for truth

Conventional financial accounting is based on double entry book-keeping, which requires every transaction to be entered twice, in different places so that the accounts as a whole remain in balance. There may be a single, definitive statement of profit, but that is distilled from multiple sources, with an intricate web of balances and documented, supporting assumptions. The whole thing is therefore verifiable, or auditable. But it’s not truth. It’s more a matter of saying “given these assumptions this set of transactions produces the following profit figure”. Vary the assumptions and you have a different and perhaps equally valid figure – so it’s not truth.

For many years academic accountants, e.g. Christopher Napier, have been doing fascinating work that strays over into philosophy. What is this reality that we are trying to understand? That’s ontology. What can we know about it, and what reliance can we put on that knowledge when we try to report it? That’s epistemology. Why are we doing it? That’s teleology.

The most interesting subject I ever studied in accountancy at university was the problem of inflation accounting. £6-£5=£1 might be a crude profit calculation for an item whose inputs cost you £5 and which you sold for £6. But what if the £5 was a cost incurred 11 months ago. You then buy replacement inputs, which now cost £7, but you’d still only be able to sell the finished product for £6. What does it mean to say you made a profit of £1? Who does that help? Couldn’t you also argue that you made a loss of £1?

What does it mean to add money together when the different elements were captured at dates when the purchasing power equivalent of that money was different? You’re adding apples and oranges. The value of money is dependent on what it can buy. Setting aside short term speculation that is what dictates currency exchange rates. £1 is more valuable than €1 because it buys more. It is meaningless to add £1 + €1 and get 2. An individual currency has different values over time, so is it any more meaningful to add different monetary figures without considering what their value was at the time the data was captured?

The academics pointed out all the problems inflation caused and came up with possible, complicated solutions. However, the profession eventually decided it was all just too difficult and pretty much gave up, except for an international standard for accounting in countries experiencing hyper-inflation (defined as greater than 100% over three years, i.e. a persisting annual rate of at least 26%). As at the end of 2014 the qualifying countries are Belarus, Venezuela, Sudan, Iran and Syria (which has rather more to worry about than financial accounting). For the rest of the world, if you want to add 5 apples and 6 oranges, that’s fine. You’ve now got 11 pieces of fruit. Stop worrying and just do the job.

I’m the treasurer for a church, and I’m often asked how much money we’ve got. I never bother going to the online bank statement, because I know that what people really want to know is how much money is available. So I use the church accounts, which factor in the income and payments that haven’t been cleared, and the money we’re due imminently, and the outgoings to which we’re already committed. These different figures all mesh together and provide a figure that we find useful, but which is different from the bank’s view of our balance. Our own accounts never rely on a single source of truth. There are multiple reconciliation checks to try and flag up errors. The hope is that inputting an incorrect amount will generate a visible error. We’re not reporting truth. All we can say is, so far as we know this is as useful and honest a statement of our finances as we can produce for our purposes, for the Church of Scotland, the Office of the Scottish Charity Regulator and the other stakeholders.

It’s messy and complex – deal with it

What’s it all got to do with testing? If your vision of testing is checking whether the apparent functionality is consistent with the specification as represented in the test script then this sort of messy complexity is a tedious distraction. It’s so much easier to pretend you can confirm the truth using a test script.

However, testing is (or should be) a difficult and intellectually demanding process of teasing out the implications of the application for the stakeholders. If you accept that, then you are far more likely to do something valuable if you stop thinking about any single source of truth. You should be thinking instead about possible sources of insight to help you shed light on the various “truths” that the various stakeholders are seeking. Understanding these different needs, and all the nuances that arise from them is essential for testers.

Assuming that there is a single truth that we can attest to with a simple, binary yes/no answer reduces testing to the level of the accountants who have tried to treat accountancy as a simple arithmetical exercise. Five oranges and six apples add up to eleven pieces of fruit; and so do eleven grapes, and eleven melons. So what? That is a useless and misleading piece of information, like the unqualified statement that the product is sound because we found what the script told us to look for. Testers, accountants and auditors all pick up good money because they are required to provide valuable information to people who need it. They should be expected to deal with messy, complex reality. They should not be allowed to get away with trying to redefine reality so it’s easier to handle.

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