Restart the program. You may be assigning more number of factors for less number of species.

Hey there Doreen Mirembe! I am here, ready to dive into the world of PMF models. I am assuming you have data for air quality, i.e. where I used. Now, if you're encountering an "exceptional error" while running your PMF model for source apportionment, here are a few things to consider:

1. **Data Issues:**

- Check your input data. Make sure it's in the correct format and doesn't contain any missing or irregular values.

- Ensure that your data matrix is well-conditioned, and there are no anomalies that could disrupt the modeling process.

2. **Model Parameters:**

- Review the parameters you've set for your PMF model. Sometimes, a small tweak can make a significant difference.

- Check if the number of factors or components you're trying to extract is reasonable for your dataset.

3. **Convergence:**

- PMF models can sometimes struggle with convergence issues. Try different initialization methods for your factors or consider adjusting the convergence criteria.

4. **Scaling:**

- Ensure that your data is appropriately scaled. Sometimes, different scales between variables can cause issues.

5. **Software-Specific Issues:**

- If you're using a specific software package for PMF, check the documentation or user forums for any reported issues or bugs. There might be a fix or workaround available.

6. **Memory and Computational Resources:**

- Large datasets might require a lot of memory or computational resources. Make sure your system can handle the size of your dataset.

7. **Outlier Detection:**

- Check for outliers in your data. Outliers can have a significant impact on factorization methods.

8. **Data Preprocessing:**

- Consider revisiting your data preprocessing steps. Sometimes, a small error in the preprocessing phase can lead to issues later.

Remember, debugging a model can be a bit of trial and error. Don't hesitate to iterate, adjust, and test different aspects to identify the root cause. If the issue persists, seeking help from the community of users for the specific software or the developers themselves might provide more insights into the exceptional error you're encountering.