To optimise an advertisement, that is displayed to a purchaser, e.g. by broadcasting the advertisement via radio or television, or by publishing the advertisement in a printed journal, the advertisement includes an address (e.g. a telephone number or an URL) identifying a server system. The purchaser instructs a client system to request a code from the server system, e.g. by sending an SMS to the server system. The server system generates a code that identifies the advertisement and causes the code to be transmitted to the client system. The purchaser purchases a product that is related to the advertisement and provides the code to the shop at which the product is purchased. The shop then causes the code to be transmitted to the server system. The server system identifies the advertisement and performs a rating of the advertisement using information related to the purchase of the product. The rating can comprise information according to the number of sales that can be assigned to the advertisement, the total sales revenue, or the average advertising costs per product. According to a preferred embodiment of the invention, an optimisation of the advertisement is performed by changing at least one parameter related to the advertisement.