The main idea is the thesis or main point of an informational text. It can be expressed anywhere in a material or paragraph, either at the beginning or middle, or at the end. If stated at the beginning, then you can expect the sentences that follow to support or develop the main idea. This is what you call deductive order. If the thesis or main idea is expressed at the end, then the earlier statements are details/specifics that build up on the main point or general statement. This text follows the deductive order.