For months, commentators had one demand of Labour: stop holding your fire and show us some policy.

Last week, Labour obliged. It would cap public transport fares at $20 a week in the big cities, and $10 everywhere else. Two numbers came attached: a cost of $65 million a year, and an average saving of $25 a week.

So, I reached for a calculator, and that is where the trouble started.

The announcement caused a bit of confusion in our office because there is more than one way in which you could read the policy.

The first reading is small. If the saving really is $25 a week, then $65 million stretches to about 54,000 people, or one New Zealander in a hundred. A modest scheme for a lucky few.

The second reading is enormous. Labour says 1.3 million people ride public transport each year, and if they all saved as promised, the bill would not be $65 million but well over a billion. On that reading, the $65 million is not a budget at all. It is a down payment.

The third reading sits somewhere between the two, and it turns on figures Labour never gave us. How many drivers will switch to the bus, and how many riders already spend enough to gain anything? Nobody outside Labour can say.

Three readers, three answers. We could not agree on what the policy was, let alone whether it was any good.

The reason is simple. Labour gave a headline and kept the workings. No model, no spreadsheet, no table of who saves what and where. In fact, not even a problem definition.

Without those, you cannot argue with the policy. You can only guess at it.

Consider one mystery. Each morning, two trains pull into Wellington station. The one from Masterton carries commuters who would save thousands a year. The one from Palmerston North carries commuters who get nothing, because their line is “inter-regional”.

Same city, same station, different outcome. Is that the plan or an accident? Nobody can tell, because the paper that would explain it was never written.

Had Labour written that paper, it would have caught all this. Writing a policy down is how a party finds out whether it works.

Commentators asked Labour for policy. But Labour only gave us a press release with a dollar sign on it.

Parties that want our votes owe us their workings, not just their answers.