I'm planning to apply to pre-docs and PhD programs in the fall and I'll be writing a senior thesis over the summer and plan on using it as my writing sample (that or a project I'm working on currently separate from my thesis). I'm wondering what admissions committees expect from this sample - rudimentary research abilities? publishable results/grad level quality?

I'm really stressed that whatever I submit won't be up to snuff. I wrote a paper for my econometrics class but it wasn't anything novel and wasn't causal, was basically just a few regressions and an analysis of those results. I'm proud of the work, but it was far from high quality academic research. I'm planning on making my thesis as polished and high quality as possible so it makes a reasonable writing sample.