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What's the question?

To answer specifically what you have posted, there is not enough information to come up with a solution.  For example, we don't know if the balls are the same size and appearance.

Balls do look exactly the same

Hi Natalya. Here goes my shot:

I believe this works like the searching for record algorithm, that's why it was given at an SQL job interview. Goes like this:

Let's call the balls 1,2,3,4,5,6,7,8.

We need to first assess what Normal and what Odd means.

Weighing 1 = (1,2 | 3,4) ("|" represents the middle of the scale :-D )

If the scale is level, then the Odd is among 5,6,7,8 and Normal is 1,2,3,4

If the scale is not level, then Odd is among 1,2,3,4 and Normal is 5,6,7,8

So now we know what a Normal ball looks like (weighs like, that is), and we know where the Odd ball is.

Let's name the group where the Odd ball is "Odd-subgroup" with balls o1,o2,o3,o4 and the "Normal-subgroup" with balls n1,n2,n3,n4.

We repeat the process with the Odd-subgrup.

Weighing 2 = (o1 | o2)

If the scale is level, then Odd is among o3,o4 and o1,o2 are normal.

If the scale is not level, then Odd is among o1,o2 and o3,o4 are normal.

Now the Odd-Subgroup has been reduced. Let's call it o1,o2 irrespective what balls they are. The other 2 balls are now n's  :-)

We repeat the process, but this time weighing ONE Odd ball against what we know is a Normal ball, for example n1.

Weighing 3 = (o1 | n1)

If the scale is level, then the Odd ball is o2.

If the scale is not level, then the Odd ball is o1.

Did I get it right, Natalya ?

PM

To simplify, I've divided the method into 3 stages: 1, 2 (a and b) and 3. It took me less than an hour to figure out :)

Stage 1: First you weigh 4 of the balls, 2 on each side of the scale. If the scale is even you know the ball is one of the remaining 4. Go to Stage 2a. If the scale is uneven, Go to Stage 2b.

Stage 2a: Then you weigh 2 of the remaining 4. If the scale is even, you know the 'odd' ball is one of the remaining 2. Go to Stage 3a below. If the scale is uneven, go to Stage 3b below.

Stage 2b: Remove two of the balls from the scale. If the scale is now even, the odd ball is one of the other 2. Go to Stage 3a. If the scale is uneven, go to Stage 3b.

Stage 3a: Remove one of the balls from the scale and put one of the remaining 2 balls on the scale. If it is now level, you know the odd ball is the one you haven't weighed yet. If the scale is uneven, you know the odd ball is the one you just put on the scale.

Stage 3b: Remove 1 of the balls from the scale and put one of the other balls on the scale. If it is now even, you know the odd one is the one you just removed. If it is uneven, you know the odd one is the ball that was left on the scale.

Three simple steps. Took me less then 3 minutes. Came to the same conclusion as vera73 but his/her post is more elegant and concise than mine would have been.