19 balls and a scale

Question

You are given 19 identical-looking balls and a two-sided scale. One of the balls is different, either lighter or heavier than the others. Using 4 weighings of the scale, how can you determine which ball is odd and whether it's lighter or heavier?(in a systematic way)

hexomino · Answer

Here is one obvious strategy

Note

A very pleasant solution to the 12 balls, 3 weighings question was referenced by Gareth McCaughan here which I have just copied into the spoiler text below for quick reference

Further reference

19 balls and a scale

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