The six criteria of effective market segmentation are: identity, accessibility, responsiveness, size, measurability, and nature of demand. Identity refers to having a means to identify members of a segment. Accessibility means the ability to reach the segment. Responsiveness is how the segment will react to marketing efforts. Size refers to the potential profitability of the segment. Measurability means being able to measure the segment's activities. Nature of demand relates to the segment's product needs.