I think the average is found by finding the average of all the sales values, but he thinks it should be found by dividing the total price for all the properties by the total floor area.

e.g. For this table:

Price (£) | GIFA (m²) | Sales value (£/m²) |

280,000 | 67 | 4179 |

280,000 | 65.1 | 4301 |

290,000 | 72.1 | 4022 |

295,000 | 75 | 3867 |

In this case my average would be 4092.25 and his 4101.003.

He thinks his average is closer to the "true average" because it ignores outliers more, but I've experimented with this and haven't seen it to be true.

Is there weight to what he's saying? Are there ways of finding an average (besides finding the median) that don't consider outliers?