1. A method of generating a handwriting profile representation for assessing the validity of a payment instrument on a computer system, comprising: providing one or more payment instruments to a computer system, wherein the payment instruments comprise at least one amount field, wherein an amount field indicates the value of a payment instrument; and defining at least one handwriting profile representation from handwriting in at least one amount field of one or more of the provided payment instruments, wherein a handwriting profile representation comprises information to assess the validity of handwriting on one or more other payment instruments.