The OCR large data set gives travel-to-work counts for 348 Local Authorities in England and Wales. A student randomly selects 20 Local Authorities and investigates the proportion of employed people who worked mainly at or from home in 2011.
State the sampling units.
State the denominator that should be used to calculate each proportion.
Explain why the number of usual residents should not be used as the denominator.
State one reason why a conclusion from the 20 selected Local Authorities may differ from a conclusion using all 348.