The article “Influence of Penetration Rate on Penetrometer Resistance” (J. Oliveira, M. Almeida, et.

The article “Influence of Penetration Rate on Penetrometer Resistance” (J. Oliveira, M. Almeida, et al., Journal of Geotechnical and Geoenvironmental Engineering, 2011:695–703) presents measures of penetration resistance for a certain fine-grained soil. Fifteen measurements, expressed as a multiple of a standard quantity, had a mean of 2.64 and a standard deviation of 1.02. Find a 95% confidence interval for the mean penetration resistance for this soil.