Basic requirements to establish that a signature is valid include:
- The signature must be of a registered voter, as established by comparing the name to that state's list of registered voters.
- The signature on the petition must match the signature on the voter registration card filed when that voter registered to vote.
- The signature must not be a duplicate signature. This occurs when a voter signs the same petition more than once. If this happens, in some states, one signature counts and the other(s) don't. In other states, if a voter signs more than once, none of his or her signatures count.
Gray areas in determining whether a signature is valid include:
- The signature is of a person who is listed as a registered voter, and the signature on the petition matches the signature on the person's voter registration card, but the address provided by the voter on the petition is different from the address provided by the person on their voter registration card.
In some states, additional factors are introduced that determine whether a signature is considered legally valid. These requirements, strictly speaking, have nothing to do with whether a registered voter signed the petition but have to do with an array of other requirements that go above-and-beyond an assessment of whether a registered voter signed a petition. These considerations include:
- Whether the circulator correctly filled out the circulator affidavit. There is a growing trend to impose many requirements on circulators and to toss all the signatures a circulated collected, regardless of whether it is reasonably possible to ascertain that the signatures on the petitions circulated by that circulator were provided by legally registered voters who intended to sign the petition to put a candidate or measure on the ballot.
- Whether petition sheet that the registered voter signed was notarized by a notary public whose notary seal was current at the time that the notary witnessed the signature of the circulator on the petition sheet.
- If signatures from voters who live in different counties are intermingled on one petition sheet. In some states, all the signatures on a petition sheet where signers from multiple counties sign are discarded. In some states, the signatures of the majority county are retained and the others are discarded. In other states, signatures count even if signatures from more than one county are intermingled on one petition sheet.
"Signer intent" is the idea that if the signature checker can reasonably conclude that a voter intended to sign a petition, that signature should count even if the signature is flawed in various ways, such as not including a date or other technical flaws.
Some states honor signer intent in checking petition signatures and other states require strict adherence to all the technical provisions the state legislature has passed governing the signature validity process.