Try switching to Python parser and using single quotes. Generally, I find Python much more stable while doing field calculations.

ArcGIS 10 Field Calculator
When performing normalization of real time PCR results, I found two ways of doing it:


In my lab they follow the next layout: $\text{Efficiency}^{-(CT\ _{\large\text{interest gene}} - CT _{\large\text{ housekeeping}})}$ each time controls with controls and treatments with treatments. Then they divide the all values (treated and controls) each with controls.
On the other hand I found another way to normalize that follows this steps: $\text{Efficiency}^{(CT_{\large\text{control}} - CT_{\large\text{treated}})}$. Then you make the same for the normalizing gene and divide the first by the last.


The values obtained are similar but not exact. Also I noticed that first algorithm gives rather different values for most diluted when performing a standard curve.
Which is correct? 
