100% of the panel saw no change, the index would be 50.0 (P2 * 0.5). Therefore, an index reading of 50.0 means that the variable is unchanged, a number over 50.0 indicates an improvement, while anything below 50.0 suggests a decline. An index of 50.0 would arise if either all respondents reported no change or the number of respondents reporting an improvement was matched by the number of respondents reporting a deterioration. The further away from 50.0 the index is, the stronger the change over the month, e.g. a reading of 55.0 points to a more frequently reported increase in a variable than a reading of 52.5. The degree of confidence experienced by respondents reporting an improvement and the degree of concern experienced by respondents reporting a deterioration are not factored into the index. Headline Manufacturing PMI The headline manufacturing PMI is a composite of five of the survey indices. These are New orders, Output, Employment, Suppliers' delivery times (inverted) and Stocks of purchases. The ISM attributes each of these variables the same weighting when calculating the overall PMI, whereas S&P uses the following weights: production (0.25), new orders (0.30), employment (0.20), supplier deliveries (0.15), and inventories (0.10). S&P Economics' PMI surveys The data for the index are collected through a survey of 400 purchasing managers in the manufacturing sector on five different fields, namely, new orders from customers, speed of supplier deliveries, inventories, order backlogs and employment level. Respondents can report either better, same or worse business conditions than previous months. For all these fields the percentage of respondents that reported better conditions than the previous months is calculated. The five percentages are multiplied by a weighting factor (the factors adding to 1) and are added. Survey panels Purchasing managers form a near ideal survey sample base, having access to information often denied to many other managers. Due to the nature of their job function, it is important that purchasing managers are among the first to know when trading conditions, and therefore company performance, change for the better or worse. Markit therefore uses such executives to produce data on business conditions. In each country, a panel of purchasing managers is carefully selected by Markit, designed to accurately represent the true structure of the chosen sector of the economy as determined by official data. Generally, value added data are used at two-digit SIC level, with a further breakdown by company