Yes it's simplified but 3quarters is still quite big part of economy, I think if we wanted more reasonable guess we would need to count what percentage of whole year gdp any quarter of the economy is.
In China case the biggest is the 4th quarter, I guess in the USA would be the same.
1st quarter is the smallest
2nd and 3rd is similar
4th is the biggest
No, it's not the size that's the difference; I'm not saying that Q1-Q3 are too small and therefore not comparable to whole years (they are but that's not my point). I'm saying that the logic of multiplying consecutive quarters using year-on-year data is mathematically flawed. For example:
You have consecutive quarters A1, B1, C1, D1, A2, B2, C2, D2 spanning 2 years.
By a quarter-on-quarter measure, you can derive your relation of B2 to A2 by simply multiplying their growth rates because quarter-on-quarter data is the amount that B2 has grown in comparison to A2, so it makes a complete story.
Now, if you use year-on-year data, you CANNOT multiply A2 and B2 because the given growth rate of B2 was NOT in relation to A2, rather it was to B1! So what sense does it make to multiply A2 and B2 if they have no relationship to each other by the year-on-year measure?
If you wanted to derive the standing of D2 in relation to A1 via quarter-on-quarter growth, you can simply multiply them together. But you wanted to derive the standing of D2 in relation to A1 via year-to-year growth, it would not be possible because year-on-year data gives no relation between quarters that are not separated by precisely 3 other quarters. With year-on-year data, you would only know the relation of D2 to D1 or A2 to A1; A and D would never cross nor any other letters.