Spelling may vary depending on how the word is used or the people who use it. Most common variant spellings differ between the British English and the American English. Some words such as neighbour/neighbor can give neighbourly/neighborly, but analysed/analyzed cannot give analyzis, since analysis is universal. It can happen to more words, but it could give a different meaning. Thus, if you are confused to which spelling to use, it is better to know its meaning first.
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