The best first step in calculating an inferential statistic in this scenario would be C. Calculate the mean for older siblings.
This step involves finding the average motivation to succeed for the older siblings in the sample group.
Understanding the mean motivation level for older siblings will provide a
starting point to compare and analyze the data between older and younger
siblings, enabling the researcher to determine if there's a significant
difference in motivation to succeed.
Therefore, The best first step in calculating an inferential statistic in this
scenario would be C. Calculate the mean for older siblings.