Nielsen Holdings Inc. is an American, information, data, and market measurement firm. Nielsen operates in over 100 countries and employs approximately 44,000 people worldwide. Mr. Ahmed is an employee of Nielsen Holdings Inc and works as Research Associate. His manager entrusts him to collect data for research by applying different types of sampling methods and techniques. Discuss the different sampling methods and techniques that might be used by Mr. Ahmed for the research work in his firm along with its merits and demerits.