Compression is the method computers use to make files smaller by reducing the number of bits (1’s and 0’s) used to store the information. Lossy compression makes the file smaller by getting rid of bits and hoping you won’t notice. In images this can be done by looking at pixels that next to each other. If they are similar, they are made to be the same. Once a file has been compressed using a lossy method, any bits that are lost cannot be recovered. With lossless compression the file is reduced without losing any quality so the original file can be restored. Generally, lossless compression will not be able to make a file as small as a lossy method.

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Bitesize

As well as the video, show students a series of pages that have been photocopied again and again. Explain to them that this is what happens if a file is compressed again and again using a very lossy method. Have students attempt to compress a series of files (image, audio or video) using a both lossy and lossless compressions. Have them compete to see who can create the smallest file size without a noticeable reduction in quality.