The time period when at least one factors is in fixed supply. Output can only be varied by using more or less variable factors with the fixed factor. With our simple production function containing only capital and labour it is usual to consider labour the variable factor and capital as the fixed factor. In other circumstances it might be land, premises or a particular category of skilled labour that is the fixed factors. Factor. The length of the short run vary from firm. For instance, a bus company might be able to extend the size of its fleet relatively quickly, particularly if bus manufacturers have unsold stock. In contrast, specialized machinery in the textile industry might take a matter of months to obtain and set up ready for production.