stable system?

[Richard White <Richard.White@Approach.Com>]

Hello,
I'd appreciate it if those more familiar with the underlying statistics and
use of control charts could advise me on the validity of my thought process
and conclusions. Let's say I have a production process, with the goal of
producing ingots of lead that weigh 302 grams each. I have taken 10 samples
off the line in sequence as follows. Each number below is an average of the
weights in the sample.
1: 285
2: 286
3: 285
4: 289
5: 290
6: 290
7: 290
8: 288
9: 288
10: 288
These average 287.90, with a standard deviation of 1.97. Taking the average
and adding and subtracting 3 standard deviations gives us an UCL of 293.81
and a LCL of 281.99.
 
My first take is that this system appears stable, but also that there is
evidence (not proof!) of some manipulation of the measurement, given that
the data just barely makes it outside +- 1 standard deviation. Such
manipulation would surprise me, though, since the target (302) is actually
higher than the average that the process is hugging so tightly.

Any thoughts? Is my methodology/math OK? Is the suspicion of manipulation
justified by the data, or is it more likely that we have simply achieved the
holy grail - i.e., highly reduced variation? If there were manipulation
going on, why might it be happening even though the average is lower than
the target?

BTW, I know 10 samples isn't very many, but it'll have to do.

Thanks in advance,
Richard White
Approach Inc.
100 Summit Lake Drive
Valhalla, NY 10595
whiter@approach.com
914-769-2800