Answer:
99% confidence interval for the difference between the mean thicknesses produced by the two methods is [0.099 μm , 0.901 μm].
Step-by-step explanation:
We are given that in a study comparing various methods of gold plating, 7 printed circuit edge connectors were gold-plated with control-immersion tip plating. The average gold thickness was 1.5 μm, with a standard deviation of 0.25 μm. 
Five connectors were masked and then plated with total immersion plating. The average gold thickness was 1.0 μm, with a standard deviation of 0.15 μm.
Firstly, the pivotal quantity for 99% confidence interval for the difference between the population mean is given by;
                               P.Q. = 
  ~ 
where, 
 = average gold thickness of control-immersion tip plating = 1.5 μm
 = average gold thickness of total immersion plating = 1.0 μm
 = sample standard deviation of control-immersion tip plating = 0.25 μm
 = sample standard deviation of total immersion plating = 0.15 μm
 = sample of printed circuit edge connectors plated with control-immersion tip plating = 7
 = sample of connectors plated with total immersion plating = 5
Also, 
   =  
  = 0.216
<em>Here for constructing 99% confidence interval we have used Two-sample t test statistics as we don't know about population standard deviations.</em>
So, 99% confidence interval for the difference between the mean population mean, (
) is ;
P(-3.169 < 
 < 3.169) = 0.99  {As the critical value of t at 10 degree of 
                                               freedom are -3.169 & 3.169 with P = 0.5%}  
P(-3.169 < 
 < 3.169) = 0.99
P( 
 < 
 < 
 ) = 0.99
P( 
 < (
) < 
 ) = 0.99
<u>99% confidence interval for</u> (
) =
 [ 
 , 
 ]
= [ 
 , 
 ]
= [0.099 μm , 0.901 μm]
Therefore, 99% confidence interval for the difference between the mean thicknesses produced by the two methods is [0.099 μm , 0.901 μm].