1. The problem is to design a sampling plan to study the monthly electricity consumption of 100 households in a city and explain data collection methods. 2. Sampling plan: Use simple random sampling to select 100 households from the city's population to ensure every household has an equal chance of being chosen. 3. Primary data collection: Collect data directly by sending surveyors to households or using smart meter readings with the consent of households to record their electricity consumption each month. 4. Secondary data collection: Obtain electricity consumption records from utility companies or public databases if available, which provide previously collected data without additional surveying. 5. To summarize, the sampling approach is simple random sampling, primary data involves direct measurements or surveys, and secondary data involves using existing records.