Questions about choices of tenses and plural/singular (scientific writting)

We assumed that the annual device scrape rate is/rate was/rates are/rates were 10% from 1960 to 1979, 30% from 1980 to 1999, and 60% from 2000 onwards.

• I know if mentioning hypotheses, we need to use present tense in the clause. Not sure if assumptions belong to hypotheses or not?
• The 'rate' refers to data in the past, should I use past tense?
• Also not sure if I should use singular or plural form for 'rate', which have different values for different time periods.

Everything is in the past, so use the past tense. Whatever the hypothesis rule is (and whatever a "scrape rate" might be), this seems to be a statement based on fact.

Whether you use a singular rate or plural rates depends on the context. Although you mention several rates, they all concern the same quantity. If at any one time there was a single rate, then use the singular. If at any one time there were several rates, then use the plural.

For example when talking about saving and loans, different companies have different interest rates at any one time, so when talking about them generally, it should be plural rates. If talking about a particular rate, that is singular. But, you might be talking about a typical rate in the industry. So it can't be said for certain whether you should use singular or plural.

However I would go for the singular.

We assumed that the annual device scrape rate was 10% from 1960 to 1979, 30% from 1980 to 1999, and 60% from 2000 onwards.