diff --git a/saqc/core/register.py b/saqc/core/register.py
index 39c1d4b14c0820f989e82672fecd1c9c9e4099c0..8cd859530713e679d5b0fdd43015ef2ce9bbbe69 100644
--- a/saqc/core/register.py
+++ b/saqc/core/register.py
@@ -238,7 +238,7 @@ def _maskData(data, flagger, columns, thresh) -> Tuple[dios.DictOfSeries, dios.D
 
     # we use numpy here because it is faster
     for c in columns:
-        col_mask = isflagged(flagger[c].to_numpy(), thresh)
+        col_mask = _isflagged(flagger[c].to_numpy(), thresh)
 
         if any(col_mask):
             col_data = data[c].to_numpy(dtype=np.float64)
@@ -250,7 +250,7 @@ def _maskData(data, flagger, columns, thresh) -> Tuple[dios.DictOfSeries, dios.D
     return data, mask
 
 
-def isflagged(flags: Union[np.array, pd.Series], thresh: float) -> Union[np.array, pd.Series]:
+def _isflagged(flags: Union[np.array, pd.Series], thresh: float) -> Union[np.array, pd.Series]:
     """
     Return a mask of flags accordingly to `thresh`. Return type is same as flags.
     """
diff --git a/saqc/funcs/interpolation.py b/saqc/funcs/interpolation.py
index 5c9e8974f517c1ed556e91cb7b760583b9bf7711..a7880f4b0a067ca98b5e3d18c4975040fbdea0e8 100644
--- a/saqc/funcs/interpolation.py
+++ b/saqc/funcs/interpolation.py
@@ -10,7 +10,7 @@ import pandas as pd
 from dios import DictOfSeries
 
 from saqc.constants import *
-from saqc.core.register import register, isflagged
+from saqc.core.register import register, _isflagged
 from saqc.flagger import Flagger
 from saqc.flagger.history import applyFunctionOnHistory
 from saqc.lib.ts_operators import interpolateNANs
@@ -248,7 +248,7 @@ def interpolateIndex(
     start, end = datcol.index[0].floor(freq), datcol.index[-1].ceil(freq)
     grid_index = pd.date_range(start=start, end=end, freq=freq, name=datcol.index.name)
 
-    flagged = isflagged(flagger[field], kwargs['to_mask'])
+    flagged = _isflagged(flagger[field], kwargs['to_mask'])
 
     # drop all points that hold no relevant grid information
     datcol = datcol[~flagged].dropna()
diff --git a/saqc/funcs/resampling.py b/saqc/funcs/resampling.py
index e64ec56f8cd46a58c1a7d5feee65079fbda30caa..0a796d726e440a544359a0f7ec084a0edf77c6f5 100644
--- a/saqc/funcs/resampling.py
+++ b/saqc/funcs/resampling.py
@@ -12,7 +12,7 @@ import pandas as pd
 from dios import DictOfSeries
 
 from saqc.constants import *
-from saqc.core.register import register, isflagged
+from saqc.core.register import register, _isflagged
 from saqc.flagger.history import applyFunctionOnHistory
 from saqc.flagger.flags import Flagger
 from saqc.funcs.tools import copy, drop, rename
@@ -393,7 +393,7 @@ def _shift(
     --------
     shift : Main caller, docstring
     """
-    flagged = isflagged(flagger[field], kwargs['to_mask'])
+    flagged = _isflagged(flagger[field], kwargs['to_mask'])
     datcol = data[field]
     datcol[flagged] = np.nan
     freq = evalFreqStr(freq, freq_check, datcol.index)
@@ -516,7 +516,7 @@ def resample(
         The flagger object, holding flags and additional Informations related to `data`.
         Flags values and shape may have changed relatively to the flagger input.
     """
-    flagged = isflagged(flagger[field], kwargs['to_mask'])
+    flagged = _isflagged(flagger[field], kwargs['to_mask'])
     datcol = data[field]
     datcol[flagged] = np.nan
     freq = evalFreqStr(freq, freq_check, datcol.index)
@@ -701,7 +701,7 @@ def reindexFlags(
         mask_kws = func_kws
 
     elif method[-5:] == "shift":
-        drop_mask = (target_datcol.isna() | isflagged(target_flagscol, kwargs['to_mask']))
+        drop_mask = (target_datcol.isna() | _isflagged(target_flagscol, kwargs['to_mask']))
         projection_method = METHOD2ARGS[method][0]
         tolerance = METHOD2ARGS[method][1](freq)
         func = _inverseShift