A ward office asked 300 people leaving the vegetable market on a Sunday morning whether the market's hours should be extended into the evening. Four in five said yes, and the office announced that the ward wants longer market hours. Which of the following, if true, would do most to show that the 300 replies can support a claim about the ward as a whole?

The doubt about the 300 is that people found leaving a market on a Sunday morning are exactly the people most attached to it, so their answers need not stand for the ward. Independent evidence from households across the ward, non-visitors included, that lands on the same four in five is what shows the narrow group happened to mirror the wide one. Uniform wording, the market's size and a full response rate all leave the group as unrepresentative as it was.