Affidavits and witness statements are similar documents but they have one important difference. The difference is not in how they are written but rather in how they are signed.
An essential part of an affidavit is that it is a statement made on oath or affirmation. An affidavit must be signed in front of a witness who is specifically authorised by the law to administer an oath or take an affirmation. The affidavit is signed by both the person making the affidavit and the authorised person at the time of swearing the oath or making the affirmation.
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A witness statement does not require an oath or affirmation to be made, and does not usually need to be signed before an authorised person. The only person who usually needs to sign a witness statement is the person making it. However sometimes the person making a witness statement will have a second person also sign the statement to witness the fact that the first person actually signed the statement (rather than somebody else). Rules or procedures in particular jurisdictions may also specify ways in which a witness statement should be signed.
Apart from this one key difference, affidavits and witness statement should be written in the same way. Affidavits and witness statements both serve the same purpose of setting out a person’s evidence in written form, and the rules for how to write such a document apply to both of them.