You are Mariana Echeverri Toro, research director at ColombiaPulse, a market research firm in Medellín. A chain of 45 hypermarkets wants to know how big the healthy-food market is in five cities before it decides on a COP 50,000 million investment. By Friday you must sign off the sample design: how many people to interview, and how to reach them.
The situation
It is Monday 2 March 2026. The study covers Bogotá, Medellín, Cali, Barranquilla and Bucaramanga, some 15 million people, and its budget is COP 120 million with no overruns: a design that costs more does not go into the field. Whatever the fieldwork does not spend is all there is for everything that comes after it. A completed online survey costs COP 40,000; a face-to-face interview, COP 120,000.
The technical proposal offers three sample sizes: 600 interviews, with a margin of error of plus or minus 4.0 points; 1,050, with 2.8; or 1,500, with 2.4. The work plan going round the team splits the fieldwork half online and half face to face.
Your team does not agree. Hernán Ospino, the field manager, wants to load almost everything onto the online survey: it is cheaper, faster and buys a bigger sample with the same money. Julián Arango, the sampling specialist, reminds him that people who answer online tend to be younger, better educated and more health-conscious than the average shopper, which is exactly the profile that buys most in this category. And the client is in a hurry: "Mariana, the Committee meets in four weeks. We need a number we can defend," Ricardo Villamizar, its new-categories manager, tells you on the phone.