The missing observations

The school weather project had run for six weeks when we began preparing the display. Two days' records were missing altogether. A third sheet listed temperatures without saying whether they had been measured indoors or outside. Different groups had used different symbols for cloud cover, and nobody had kept a key. Some entries gave the time as 'after lunch', although lunch had moved during the examination week. We could not repeat those past observations. Instead, we marked the gaps and explained why parts of the chart could not fairly be compared.

Describe the problems with comparing the weather records. You may support your points with brief quotations.