Final answer:
The quality-control manager at Intel is using systematic sampling, as she selects every 8th chip starting with the 3rd unit until obtaining a sample of 140 chips. This method is characterized by a regular interval selection (every 8th chip), which differentiates it from other methods like simple random, stratified, cluster, or convenience sampling.
Step-by-step explanation:
To identify the type of sampling used by the Intel quality-control manager when she selects every 8th chip that comes off the assembly line starting with the 3rd unit until she obtains a sample of 140 chips, it is important to understand the definitions of different sampling methods.
Systematic sampling is a method where the researcher selects every kth individual from the population, the value of k being systematically determined beforehand. The starting point is usually chosen randomly within the first interval. In this scenario, the manager is indeed selecting every 8th chip, which is a systematic procedure starting from the 3rd unit, thereby it is systematic sampling.
This differs from simple random sampling, where each member of the population has an equal chance of being included in the sample, and no regular interval is followed. Stratified sampling involves dividing the population into strata and randomly selecting from each stratum; this is not indicated here as there is no mention of different groups or layers from which the sample is drawn. Cluster sampling entangles selecting entire groups or clusters randomly, which is not the case here either. Lastly, convenience sampling involves choosing individuals most accessible or convenient to the researcher, which also does not apply in this context.
Therefore, the correct option for the Intel quality-control manager's strategy to estimate the percentage of defects is systematic sampling.