The figure calculated will vary according to whether it is based on the value of sales or the volume of sales. To illustrate this, consider the likely market share for Marks and Spencer. It should be fairly obvious that a calculation of market share based on the value of sales would give a much higher figure than one based on the volume of sales. This is because Marks and Spencer is likely to sell significantly less groceries than firms such as ASDA. However each unit sold by Marks and Spencer is likely to be of a much higher value. When trying to measure the market share of a company like Tesco, you need to consider whether the market you are measuring is for food, groceries or general consumer goods. You also need to consider whether the market you are measuring is the local market, the regional market or the national market. For example, while Tesco has a UK national market share of approximately 30.4%, its market share may be significantly larger in some smaller geographical areas