Data Science and Machine Learning hiring managers are looking for specifics skills* when considering PhDs right out of graduate school:
- An ability to problem solve when you only have partial information.
- Some demonstration that you've taken independent steps to learn the field. Usually this is the basics for a handful of machine learning models and, more importantly, their mathematical underpinnings.
- A good attitude/team fit.
No certificate gives you those things, except maybe an online course supporting that you worked towards number 2. A strong track record of public speaking never hurts, but interactions during the interview will dominate the evaluation of these skills.
On the flip side, some big red flags for "data science" positions are requirements for Microsoft/Oracle/similar certifications. While data science is not rigidly defined, I am starting to see** the title given to data engineering positions, devops positions, and other similar programming roles that have a splash of statistics in the requirements. If you want to be doing research and applying machine learning models to business use cases, avoid these postings like the plague.
*Unless you are applying for a research position with an ML degree, in which case this does not apply.
**I am US based, not German, so not sure what differences that entails.