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In Task 1, is it better to write summary in starting paragraph or at the end as a conclusion

Either is OK. In terms of task achievement, for academic writing part one, to get band 7 it is critical to address all the major points made in the charts, tables, maps or illustrations, and to give a "clear overview". Leave out any one major point, and you are restricted to band 4. There is one more pitfall to avoid if you want to do better than band 5 in TA; use accurate data to support your points. Failure to provide an overview you will restrict to band 5 or less for task achievement. If there is an overview but the examiner feels it is not "clear" you can't rise above 6. To give a clear overview it's necessary to do two things: describe the graph/table/chart/map and also summarise the main features, or trends, in one or two sentences. You could summarise the main trends at the end, or at the beginning, but I recommend you do both in the first paragraph.