Häufigkeit vs. Frequenz
The German words Häufigkeit and Frequenz both translate to "frequency" in English, but they belong to entirely different contexts. The word Häufigkeit is a broad, everyday term used to describe how often an event occurs, while Frequenz is a specialized term reserved for scientific, technical, medical, or rhythmic measurements.
Häufigkeit
The word Häufigkeit describes the general rate of occurrence of an event and is predominantly used in everyday language, demographics, and statistics to express how often something happens over a period of time.
Die Häufigkeit von Naturkatastrophen hat in den letzten Jahren leider zugenommen.
(The frequency of natural disasters has unfortunately increased in recent years.)
Wir untersuchen die statistische Häufigkeit dieses Nachnamens in Deutschland.
(We are investigating the statistical frequency of this surname in Germany.)
Er war ein wenig überrascht über die Häufigkeit ihrer spontanen Besuche.
(He was a little surprised by the frequency of her spontaneous visits.)
Diese seltene Krankheit hat eine sehr geringe Häufigkeit in der gesamten Bevölkerung.
(This rare disease has a very low frequency in the entire population.)
Die absolute und relative Häufigkeit der Fehler wird im monatlichen Bericht analysiert.
(The absolute and relative frequency of the errors is analyzed in the monthly report.)
Frequenz
The word Frequenz is a technical, medical, or scientific term used to measure strictly cyclical events per unit of time, such as sound waves, radio signals, heartbeats, or the regular intervals of public transport.
Der lokale Radiosender sendet auf einer völlig neuen Frequenz.
(The local radio station is broadcasting on a completely new frequency.)
Der Kardiologe überprüfte während der Operation die Frequenz ihres Herzschlags.
(The cardiologist checked the frequency of her heartbeat during the surgery.)
Wir müssen die genaue Frequenz des elektrischen Stroms mit diesem Gerät messen.
(We need to measure the exact frequency of the electrical current with this device.)
Das menschliche Ohr kann diese extrem tiefe Frequenz normalerweise nicht wahrnehmen.
(The human ear usually cannot perceive this extremely low frequency.)
Die Pendlerzüge auf dieser Strecke fahren morgens mit einer viel höheren Frequenz.
(The commuter trains on this route run at a much higher frequency in the morning.)
Summary
In short, you should use Häufigkeit when you want to talk about how often everyday events, habits, or statistical data points occur. You must switch to Frequenz when you are dealing with physics, medical vitals, or technical measurements that strictly count cycles or intervals per specific unit of time.