Consider the age of the patient. Most physicians advise a rectal thermometer for infants, but advocate for other types of models for older people to lessen the risk of injury that can come from movement with a rectal thermometer. If the person whose temperature you're taking is prone to moving or has a lowered ability to stay still during a reading, a less-invasive thermometer or one that provides quicker results could be the answer.
When do readings usually take place? For a sick newborn, parents may want to take temperature readings during the night to help the infant rest, while older children and adults may have their temperatures taken during the day. Several aspects of thermometers, including how invasive the thermometer is and how loud a confirmation sound is can affect whether a person wakes up from a reading.
Do you need to document temperatures over time? If you're using a digital thermometer for fertility tracking or for a chronic illness that requires ongoing documentation, a thermometer with a larger storage capacity is more convenient. Some digital thermometers store a greater number of readings, such as 20 or 30, so you don't have to document constantly, something that can be especially helpful when trying to conceive.
Will you be using the thermometer for more than one person? If you're looking for a digital thermometer for the whole family, look for a model with disposable probe covers or an easy-to-clean probe. Thermometers that can take readings from multiple body parts, such as under the arm and orally, are more versatile for multi-person use.