How we put a guide together

1. Build the long list

We start with what people actually buy. That means the best-seller lists at the big retailers in each country, the models owners talk about on forums and the brands that turn up again and again in search. A typical long list has 15 to 25 products.

2. Cut it down

We drop anything that is hard to buy, has been replaced by a newer model, or has a pattern of serious complaints in owner reviews. We also drop products whose makers will not say what parts cost. If you cannot find the price of a replacement filter, you cannot work out the cost to own.

3. Do the sums

For every product on the shortlist we work out:

• Running cost. Rated power in kilowatts, multiplied by the hours of use we assume, multiplied by the electricity rate. In the UK we use the Ofgem price cap unit rate in force on the day we check. In the US we use the latest national average residential rate from the Energy Information Administration.
• Consumables and parts. The maker's own price for filters, heads, pads or descaler, multiplied by how often the maker says to replace them.
• Lifespan. The warranty length, and what owners report in long-term reviews. Where these disagree we use the shorter figure.
• Cost to own. The price today plus the running and parts costs over a set number of years, stated on every pick.

Our assumptions are written into each guide. If you use a product more or less than we assume, your number will be different, and you can scale it.

4. Read the evidence

We read the specifications, the manuals and the warranty documents. We pool owner reviews from several retailers and look for faults that come up again and again. Where independent labs or consumer groups have published test results, we cite them by name. We do not repeat a claim we cannot trace to a source.

5. Rank and rate

The rating out of 10 weighs how well the product does its job, what it costs to own, how long it should last and how easy it is to live with. The order of a list follows the rating. Price alone never decides first place, and neither does a commission.

Keeping guides current

We re-check prices every month and re-read each guide in full at least every six months, or sooner when a new model arrives. The "updated" date at the top of a guide shows the last full check.