systematisch
systematically
In a planned, orderly, and methodical way that follows a fixed system or plan.
Die Polizei ging bei den Ermittlungen sehr systematisch vor.
The police proceeded very systematically in the investigation.

systematic
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systematically
In a planned, orderly, and methodical way that follows a fixed system or plan.
Die Polizei ging bei den Ermittlungen sehr systematisch vor.
The police proceeded very systematically in the investigation.

'Systematisch' (systematic/systematically) is derived from the noun 'das System' (the system). It describes something that follows the logic of a system, meaning it is orderly and planned.
Für eine effiziente Arbeitsweise braucht man ein gutes System.
For an efficient way of working, you need a good system.
'Systematisch' (systematic/systematically) can describe an action as an adverb (How is something done?) or stand before a noun as an adjective and describe it.
Sie hat ihr Zimmer systematisch aufgeräumt.
She tidied her room systematically.
In its general meaning, 'systematisch' (systematically) is often used with verbs of investigating or organizing, for example 'systematisch vorgehen' (to proceed systematically), 'systematisch suchen' (to search systematically), or 'systematisch analysieren' (to analyze systematically).
Die Detektive durchsuchten das Haus systematisch nach Spuren.
The detectives searched the house systematically for clues.
The general meaning is 'planmäßig' (according to plan/systematic). In scientific texts, e.g. in biology or linguistics, 'systematisch' (systematic) can also mean that something belongs to a classification system.
Er hat seine Briefmarkensammlung systematisch geordnet.
He organized his stamp collection systematically.
planned
'geplant' and 'systematisch' both describe that something does not simply happen spontaneously or by chance. 'geplant' emphasizes that you decided or prepared in advance what should happen. 'systematisch' places more emphasis on the ordered method: you proceed step by step according to a fixed plan or system. A party can be planned without necessarily being organized systematically. With learning, searching, checking, or working, 'systematisch' often sounds more precise when order and method are important.

Lea prüft die Rechnungen systematisch, damit sie keinen Fehler übersieht.
Lea checks the invoices systematically so that she does not miss any mistakes.
Die Reise war lange geplant, deshalb hatten wir schon alle Tickets.
The trip had been planned for a long time, so we already had all the tickets.