How many animals need to be weighed?
To get an accurate mob average there is no need to weigh
every single animal in the mob. In fact, it may not even be necessary to weigh
half, a quarter or even 10%. It all depends on the range of weights in the mob.
To be 95% confident of mob average within 10kg it takes:
- 20 animals if the weight range is 100kg
(lightest to heaviest)
- 35 animals if the weight range is 150kg
- 65 animals if the weight range is 200kg
Interestingly, we have learnt that different animals
typically visit the Optiweigh unit from day to day. Accordingly, we use an
average of all the animals that have visited in the past five days to calculate
the mob average. This means that 10-20 animals
per day can give a five day average comprising 40-60 or more animals – more
than enough for highly accurate data, even for a very large mob.
With sampling there is always the chance of error - for example if the group visiting the Optiweigh are the larger animals in the mob. While we have seen this it is very rare and generally happens when mobs have been mixed (for example - a mob of heavier animals who are familiar mixed with a mob of lighter animals who are not familiar with it).
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