To increase the external validity of our findings we used a populist sampling frame to identify both the recipes and the ready meals . However , the nutritional content of recipes varied substantially between individual recipe books ( data available from the author ) , suggesting that a different selection process may have led to different findings . Selecting books that were bestsellers in the run - up to Christmas may have influenced the selection of recipes , and the transient nature of bestseller charts may challenge the representativeness of the sample . The size of the sample prevented subgroup analyses comparing individual chefs or supermarkets .