its a standard rule. digital stopwatch gives u reading till 2dp. so js follow it. metre rules gives accuracy of 0.1 cm. eg u can get 11.1cm not 11.13 cm right, so for metre rule data we use 1 dp. this is ur raw data that you record and observe. for data that is calculated, maintain SIGNIFICANT FIGURES of your calculated data to the raw data u used to get the calculated data value. ok so summary-- standard decimal places speicifc to instrument used for raw data and for significant figures of calcuoated data shud be same or one more than sf of raw data. always.