Were completing
Were completing??? It is a describition of a pie chart about the change of situation of women education. As we known, It's static and happened in the past. How can it work? Thanks.Text: In 1945 only 30% of women completed their secondary education and 1% went on to a first degree. No women had completed post-graduate studies. This situation had changed radically by 1995. In 1995, 90% of women in Someland had completed secondary education and of those, half had graduated from an initial degree and 20% had gone on to postgraduate studies. At the other end of the scale we can see that by 1995 all girls were completing lower secondary, although 10% ended their schooling at this point. This is in stark contrast with 1945 when only 30% of girls completed primary school, 35% had no schooling at all and 35% only completed the third grade.

Question

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it is a past continuous tense.

Answer

Past continuous indicates something happening in the past but not finishing.

Compare: 
1 In 1995 all girls completed school.
2 By 1995 all girls were completing school.

The text is describing progress in girls' education; sentence 1 tells us only about 1995, but nothing about later years. In context sentence 2 tells us the same about 1995, but also that this continued subsequently.

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the dates just show a fact that the women's rate of education. How can the writer know they were completing their schooling rather than have completed ?

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What do you mean by static, and how is that relevant? 
It is certainly correct. If you don't understand it, you should tell us why you think it is wrong, so that we can help you to understand, or correct your misapprehensions.

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Contrast, perhaps
are completing
and
were completing
.
Today someone is completing this answer.
Back in, 1995 they were just completing lower secondary school. (Completing took some time)
By 1996, maybe they had completed lower secondary school.