You've prompted me to have a look, I haven't thought about this before.
There seems to be a lot more to it than appears at first glance.
Employer seems to be coded as 2.
Own account is coded as 6
Own account at home is coded as 3 or 7 (but 3 may be employer at home where the employees are also living there)
Apprentice is coded as 8
Worker at home is sometimes coded as 5 and sometimes as 1. I think the difference may be connected to the differences between 4 and 0.
Worker is coded as either 4 or 0
It appears that whether a worker is 4 or 0 seems to depend on what industry they work in (although it isn't always consistent, it appears mistakes were sometimes made in the coding).
So, all jobs that were connected with domestic service or cleaning were coded as 0. So, this would be cook, housemaid, gardener, char woman etc
Code 0 was also used for jobs like solicitors, teachers (and other education employees), vicars (and priests, nuns, monks etc), police officers, post office workers and firemen.
There was a training school for girls where the Sisters in charge ("Sister of Mercy") were code 0 and the matrons (referred to as "Industrial Home Matron") were code 4
But it was also used for specific industries as well. All coal miners and others working in the coal industry were code 0 as were people who worked for railway companies.
(As an example, I have relatives on the same census form, one worked in a factory making railway wagons and the other worked for a railway company that used those wagons. The person who made the railway wagons was coded 4 and the person who worked for the railway company was coded 0)
Farm labourers were all coded 0 (not the actual farmer who would be either an employer or own account).
Dock workers were code 0
Then there were some odd ones. Those with an occupation of general labourer with a code of 330 were also coded 0
I also found people working in millers and flour/seed merchants (occupation code 923, 924) who were also coded as 0.
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I'm sure that there will be more examples as well that I haven't come across.
It would be really interesting to find out if there is a definitive list somewhere of which occupations were coded 0.