A company has produced a batch of 1000 CPUs whose clock speeds follow a normal distribution centred around 2.1 GHz, with a standard deviation of 0.4 GHz. The company is trying new approaches to manufacturing CPUs, therefore 20 of these CPUs were produced with an additional new experimental feature. The clock speed of these 20 experimental CPUs is as follows (in GHz): 2.5, 1.7, 2.9, 2.7, 1.4, 1.9, 1.3, 1.9, 2.8, 2.2, 2.4, 1.7, 1.8, 2.2, 2.2, 1.9, 2.7, 3.3, 1.9, 2.1.

Design and perform a statistical test to check if the difference in clock speed for this new experimental technology is statistically significant, or if the difference is just due to chance. Hints: use a one-sample t-test, specifying the assumptions and finding the p-value.

What additional assumption would be needed to use the z-test instead?